I don't want gisters to accidentally create public gists of stuff that they thought would be private.
This commit gives jist the new ability to shorten gist URLs using GitHub's URL shortener, git.io. It adds two options on the command line (`-s` and `--shorten`). If the shortening step fails, or if git.io is down, jist will just return the normal, unshortened version.
We investigated using the clipboard gem but it was deemed to be unnecessarily complicated, and made assumptions that we'd rather not be lumbered with.