Update formatting in compliance with pep8 with the following exceptions:
* truncate to 120 characters in line
* prefer double quotes `"` to single quotes `'` in strings
Switch addon lookup mechanism from parsing html page to extracting info
from .gitmodules file.
This simplifies logic and allows using non-Github repos.
Readme for Github repos are extracted from HTML pages using regex.
Gitlab pages are converted to HTML using Python Markdown lib if present,
falling back to displaying raw markdown.
In this case image links are converted from relative to absolute paths.
Make the Addon Manager GUI friendlier by adding more color and uniformity to the UI for Addons that have a: pending-update, no-update, or are obsolete.
