vim-plugins: rewrite updater

A new python script has been added to replace the aged viml-based
updater. The new updater has the following advantages:

- use rss feeds to check for updates quicker
- parallel downloads & better caching
- uses proper override mechanism instead of text substitution
- update generated files in-place instead of having to insert updated plugins manually

Automatically reading `dependencies` from the plugins directory has been
not re-implemented.
This has been mostly been used by Mark Weber's plugins, which seem to
no longer receive regular updates.
This could be implemented in future as required.