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}-*`
I think you can do:

``` bash
${CTAGS:-"ctags"} args
```

(instead of just `ctags`)

This will allow users to set a `CTAGS` environment variable to override.

ping @pbrisbin