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've run it manually, but only in edge cases. It could be inlined but also doesn't really hurt.

Running it manually means you don't need to re-checkout to get updated ctags.