Git: set up automatic ctags

Implement Tim Pope's "Effortless Ctags with Git" approach to
(re)generate tags for the current repository via git hooks.

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

The `git init` command will copy git hook templates from
init.templatedir into the .git/ of a given repository.
These hooks will keep the tags up-to-date as the repository changes.
Tags can be manually regenerated with the `git ctags` alias.