Add options for github URL and print-only.

* Use argparse for real cli parsing.
* Add github option to print compare URL and open in browser.
* Add print-only option to only print log or github URL.
* Modify setup.py to support develop mode.

I have a separate utility in bedrock for generating the github
compare URLs that I could delete with these changes. My util
will also use the github API to grab the latest SHA for a branch,
but I don't know if that's very useful since the local SHA should
work in most cases. It's easily added as an option if you want.