Added Plugins page

The data is generated from our plugins json file which is still a bit
impure so some hacks have been placed to beautify it.

These won't be needed once our Gen JSON script improves and can parse
the data from .py files correctly.

Currently, the link to the plugins page is a bit hidden, but we can add
additional links later.
Merge pull request #5 from musicbrainz/plugins

Added a Plugins page.