Add git_template directory with base ctags hook

Based on Tim Pope's Effortless Ctags, added a git_template directory
to prepopulate new and cloned repos with hooks to run Ctags on a project
on merge, commit, and checkout.

The functionality requires Exuberant Ctags but degrades nicely, failing
silently in the absence of Exuberant Ctags (either completely absent or
OS X default ctags).