Reset on max expansion prototype

Hi Aron.  Thanks for building this plugin!

Here's a quick prototype of resetting on max expansion.  It doesn't cover the multiple-selections use-case, but it's one potential way to handle de-expansion from #31 .  Caveat - this is my first attempt at working on a sublimetext plugin, and my experience with python is limited.  Hopefully it's a decent starting point for thinking about the problem.

Note on resetting: I initially tried to reset initial location whenever the previous command in history was not expand_region, but command history seems to be super unreliable in SublimeText3.

Overall, I'm super grateful for the time you put in on this.  Thank you.  A dedicated key with expand_region is so much faster than multiple commands!