Add Indent Guides plugin

Visually displays indentation so you can have an easier time seeing
where a block ends.

Like BBEdit's tab stops (kind of), but looks more like the Visual
Studio style (blech).

Going to be amazingly useful since my team enforces 2-space indentation
for all front-end code (WTF???!).
Update CHANGELOG for Indent Guides plugin
vimrc: add settings for Indent Guides

* Remap toggle to <leader>ts
* Set column size to 1
* Set colours of guides to blend better with the background and be less
  distracting