add clone feature (shells to git)

Here's a basic implementation of the clone subcommand (see Issue #8 https://github.com/jdevesa/gists/issues/8).

This takes a minimal approach and just shells out to `git` to perform the clone with the format of `git clone git@github.com:<gist_id>.git`, which uses the user's already configured SSH config. It supports the `-o OUTPUT_DIR` option as well (by default it will go in current directory, with a subdir named for the gist id, which is git's default clone behaviour).

Hope this helps!