no matter if theme argument is a URL, eventually we should set `theme` according to `newdir` because that is the actual folder name

this is a fundamental fix to #291

* replace 'yihui/hugo-lithium-theme' with 'yihui/hugo-lithium'

When I run `blogdown::new_site()` with the default `theme = "yihui/hugo-lithium-theme"`, I get an error:
```
Rendering content/post/2015-07-23-r-rmarkdown.Rmd
Error: Unable to find theme Directory: /Users/rnolan/Dropbox/R/blog/themes/hugo-lithium-theme

Serving the directory /Users/rnolan/Dropbox/R/blog at http://127.0.0.1:5641
To stop the server, run servr::daemon_stop("4326078320") or restart your R session
Error in setwd(site.dir) : cannot change working directory
```
However I notice that even though there's no `/Users/rnolan/Dropbox/R/blog/themes/hugo-lithium-theme`, there is a `/Users/rnolan/Dropbox/R/blog/themes/hugo-lithium`. For reasons that I don't fully understand, when I run `blogdown::new_site(theme = "yihui/hugo-lithium-theme")`, everything works fine. This may be because https://github.com/yihui/hugo-lithium-theme redirects to https://github.com/yihui/hugo-lithium, but I don't know. In any case, https://github.com/yihui/hugo-lithium seems to be in working order and I think it should all work fine with that as the new default.
Thanks Yihui and other authors and maintainers, your work makes our lives easier :)

* Address CI issues.

* Undo unnecessary changes in PR (the necessary ones are still there).

* Try to keep Yihui's style for everything.

* rename more instances of hugo-lithium-theme to hugo-lithium

* use Rd2roxygen::rab()

* bump version

tweaking after #288 (overload the first argument of install_hugo() so that users can provide a local zip/tar.gz)

* Add documentation on blogdown.hugo.dir

* hash not necessary

* hash not necessary

* Describe the insert image addin as requested in https://github.com/rstudio/blogdown/pull/272#issuecomment-375790771

* minor tweaks

This suppresses the warning call trace as it appears to show that it is inside of `in("themes", {` which may confusion as `install_themes()` is what the user calls. This also matches the use `stop()` in the next `if` statement.

* Another attempt to resolve https://github.com/rstudio/blogdown/pull/275

* Add in changes from https://github.com/lcolladotor/blogdown/commit/471b086eba4e3dffc0c828a0465950f643361aa0#diff-e9d65be142465778caf366e1b7a25ae5R24

* Revert "Add in changes from https://github.com/lcolladotor/blogdown/commit/471b086eba4e3dffc0c828a0465950f643361aa0#diff-e9d65be142465778caf366e1b7a25ae5R24"

This reverts commit 96324ba09b23803e109940b18347b32a31929be5.

* Make the option 'stealthy'

* Insert image in format consistent with pandoc

* Use pandoc syntax in insert image addin for Rmd post

* Checks w/h formatting for images inserted into Rmd posts

Removes spaces and issues warning if format adjustment needed

* Safely validate CSS unit with htmltools::validateCssUnit

* Return original css unit in safely_validateCssUnit error handler

* Only check if source extension is Rmd

* No need for return statement

change gsub() to sub() because we only want to substitute the first instance of tmpdir in zipdir; add fixed = TRUE because we only want to substitute the fixed string

fix #273: the path of hugo found from bin_paths() could be the same as ~/bin/hugo on Linux if ~/bin is in PATH

* v0.5.7 -- adds insert external image addin. Resolves https://github.com/rstudio/blogdown/issues/269

* Address PR requested changes

* Address second round of requested changes

* & -> &&

* rename blogfile to ctx, and use it as the explicit id in insertText()

* Update addins.dcf

* Allow full URL in theme arg in new_site and install_theme (#270)

* Unzip theme into tmpdir to respect force arg

* Strip tmpdir from zipdir after finding expdir

* Refactor zipdir/newdir in install_theme

* Clean SHA and branch from theme install dir

* Hard OR if branch is empty string

* Add contributor and news item for theme arg update

* v0.5.5 -- resolves #173

Details: default_kind() now finds the archetype matching the type of content. The new_post addin now lets you choose an archetype.

* v0.5.6 new_content() can now work with files that end in .Rmd and .Rmarkdown. Resolves #261.

Details: new_content() will create a copy of the file with a .md termination so that hugo will be able to work with it. Once hugo is done, the temporary files are deleted.

* Add @lcolladotor as a ctb

* Fix  menu location to keep height at 500 px

* No longer assume that the path includes /content/ for default_kind()

* Remove the big if() {} else {} code. However some if()s are still needed in case that the file does indeed have a .md file extension (otherwise it gets deleted). Hopefully this look less complicated

* Switch to code @yihui wrote in the PR review (after testing it)

* Switch back to original code for default_kind() now that I understand correctly that path is post/date-slug.md or something like that instead of content/post/date-slug.md

* adds host to formals of install_theme() and new_site()

* changes `host` to `hostname`

* adds hostname functionality

* rebuilds Rd files

* moves hostname next to theme in formals

* adds @ijlyttle to contributors

adds bullet point to NEWS.md (corrects typo in previous bullet)

the method='html_encoded' has been removed, and the HTML comments in the Pandoc template no longer make sense