Map vim leader to <Space>

I read about people preferring this to comma, and figured I'd give it a
shot.
Map <Leader>w to save file in vim

This might save me some effort.
Map <Leader><Leader> to enter visual line mode in vim

I find myself always using visual line mode. This mapping might save me
some effort.
Add vim-expand-region plugin

At the same time, I mapped this to v and <C-v> for expand and shrink,
respectively.

Inspired by:

  http://sheerun.net/2014/03/21/how-to-boost-your-vim-productivity/
Add vim-repeat plugin

This claims to make my surround plugin repeatable using "." in vim. I'm
intrigued.
Add vim-github-comment plugin

This allows me to comment on commits on Github straight from vim.

   ... just run :GHComment <your-comment-here>. This will create a
   comment on the commit that changed that line last.
Add splitjoin vim plugin

This plugin makes it easy to change single-line code to multi-line code,
and vice versa.

I mapped sj to split and sk to join, as recommended by the plugin
documentation.