Introduces `git_template` for init templating

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

* Make `ctags` executable configurable
* Adds `git ctags`
* Runs `ctag` on `commit` / `branch` / `checkout`
* Extensible via `~/.git_template.local/hooks/{pre,post}-*`
* explain `git_template` under `git` section