Improve ag tab completion

* Remove unnecessary `uniq`
* Look for tags in tmp/tags or .git/tags
* Send errors to /dev/null if the file(s) do not exist

See http://tbaggery.com/2011/08/08/effortless-ctags-with-git.html for an
explanation behind the reasoning for .git/tags.
Add vim-multiple-cursors plugin for Vim
Revert "Add vim-multiple-cursors plugin for Vim"

This reverts commit 422a6f6d3324a7c6060793657054025d41a7bab3.

Want to go through a PR cycle.
Introduces `git_template` for init templating

Inspired by http://tbaggery.com/2011/08/08/effortless-ctags-with-git.html

* Runs `ctag` on `commit` / `branch` / `checkout`
* Extensible via `~/.git_template.local/hooks/{pre,post}-*`
More thoroughly explain `git_template` under `git`