Add support for themes in subdirectories

* Themes can now reside in $ZSH/themes/<theme_name>, in which case the theme
  entry point should be $ZSH/themes/<theme_name>/zsh-theme.

* If the theme is in a subdirectory, then the environment variable
  ZSH_THEME_DIR will be set to the directory where the theme resides.
Add badwolf vendor submodule
Create badwolf theme

* Theme is not yet complete, still requires some more work.
Merge branch 'feature/themedir' into develop
Clean up badwolf highlight theme
Update zsh-syntax-highlighting module
Merge branch 'feature/newthemes' into develop
Bump version number
Update zsh version information in documentation
Merge branch 'release/0.6.3'