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}-*`
Making `ctags` configurable (with a default of "ctags") seems easy enough, so I agree with that.

I'm guessing most languages, including ObjC, Swift, and Haskell, get indexed on their source files and not generated files. If not, I think this is a good enough place to start, and we can improve it with future pull requests.