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.

doc/languages-frameworks/vim.section.md

@@ -179,6 +179,18 @@ Sample output2:
     ]
 
 
+## Adding new plugins to nixpkgs
+
+In `pkgs/misc/vim-plugins/vim-plugin-names` we store the plugin names
+for all vim plugins we automatically generate plugins for.
+The format of this file `github username/github repository`:
+For example https://github.com/scrooloose/nerdtree becomes `scrooloose/nerdtree`.
+After adding your plugin to this file run the `./update.py` in the same folder.
+This will updated a file called `generated.nix` and make your plugin accessible in the
+`vimPlugins` attribute set (`vimPlugins.nerdtree` in our example).
+If additional steps to the build process of the plugin are required, add an
+override to the `pkgs/misc/vim-plugins/default.nix` in the same directory.
+
 ## Important repositories
 
 - [vim-pi](https://bitbucket.org/vimcommunity/vim-pi) is a plugin repository