Implement #179

First shot at enabling installation of theme via a url, specified by the
config param `theme_url`.
`theme_url` should point to a zip file online. The zip file will be
downloaded and unzipped to the `themes` folder.
Note that as of current implementation, `theme_url` installs the theme,
but does not use it, i.e. the `theme` parameter needs to be specified in
order for the newly installed theme to be used.

A point of consideration:
    - Should we check if the theme already exists? This is to prevent
      downloading every single time we run `config.py`
Test to support `theme_url` implementation

This test isn't that useful, and refers to a hardcoded url in one of my
branches.
Might want to consider if this test is even needed.
Ref #179