Introduces `git_template` for `git 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}-*`
Can `ctags` be configured by an env var? I know Debian calls it something else, for example.

Can the flags to `ctags` also be configured?

Using `git ls-files` means that many generated files will not be tagged. This is fine for Rails; does this matter for gitsh, ObjC, Swift, Yesod?