Make HEAD installs of tig possible

This patch adds the ability to install tig, with man pages and html docs, via HEAD.

A custom download strategy was needed to make the documentation work, since it's just using the latest release's docs..versus having to have a ton of dependencies and also having to create a new formula for one of those dependencies.

This is probably the most complicated patch I've created, so any feedback on changes to make and whatnot are greatly appreciated.