Adds Bash completions file

[jamessimas]

This PR piggybacks on the work started in https://github.com/tomnomnom/gron/pull/113 (which seems to have stalled out) to add a file which performs Bash completions (https://github.com/tomnomnom/gron/issues/112).

Here's an example of me using it locally:

[21:16] widget ~ $ gron --
--colorize    --insecure    --json        --monochrome  --no-sort     --stream      --ungron      --values      --version
[21:16] widget ~ $ gron --