github-commits: Add HUBOT_GITIO to make Git.io shortening opt-in

Short URLs are nice, but when I don't have a browser running, I prefer
something with the unadulterated commit hash.  With the new
envorinment variable, you can turn Git.io shortening on or off as you
see fit.

The alternative URL manipulation and HUBOT_GITHUB_API docs come from
github-commit-link.coffee.
github-commits: Add HUBOT_COMMIT_ONELINE to only show the summary line

Following 660ae28 (github-commit-link: Only show the first line of the
commit, 2014-03-13), but configurable this time, for folks who push
tiny feature branches and *do* actually want the whole message dumped
into their channel.