Add support to user less/precompiled themes

[osamaar]

Hey!

These changes add the ability to use local themes from user's home directory the same way packaged themes are used. The directory structure of site-packages/jupyterthemes can be reflected in ~/.jupyter-themes and extra/modified styles can be dropped there.

I also added the functionality of installing themes available in compatible css from either user (if user config dir exists) or packaged themes directory.

If both .less and .css versions of a certain theme exist, the .less version is used. If a theme exists as both a user theme and a global theme, the user theme is used. This came in handy when I wanted to slightly modify one of the packaged thems, I copied the .less file with the same file name to my own user themes directory and modified it without touching the one installed with the package. Now I can switch themes and keep my customizations (no need to edit jupyter's own custom.css).

The following demonestrates the usage with the new functionility. Notice that oceans16.less file exists in my user configuration directory and overshadows the original in jt -l. Also notice the file jdark.css which is a css only theme i got from the repo: powerpak/jupyter-dark-theme. You can see the result of installing it in the screenshot below.

$ jt -l
Available Themes: 
   chesterish    (global)
   grade3        (global)
   jdark         (user, precompiled)
   oceans16      (user)
   onedork       (global)

$ tree ~/.jupyter-themes
/home/me/.jupyter-themes
└── styles
    ├── compiled
    │   └── jdark.css
    └── oceans16.less

2 directories, 2 files
$ jt -t jdark
$ jupyter-notebook

Result: