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 it's useful to have the alias available generally. I've found that if I've written something new but haven't checked in the code yet that it's occasionally useful to be able to rebuilt the tags. The alternative could be to make it a shell alias rather than a git alias (which is what I'm doing here): https://github.com/geoffharcourt/dotfiles/blob/master/aliases.local#L53