quick toggle last session

[dbachelder]

I was going to add this to my fork, but since you're re-writing things I think it would be really nice to have a quick switch to the last open session.

1. in plugin select session A
2. in plugin select session B
3. open plugin - hit enter - goes back to A
4. open plugin - hit enter - goes back to B
5. etc..

thoughts?

[dbachelder]

I feel like there must be a better way to do this.. but this is what I got working for the described flow (it's a PR against the changes I made in my other PR to this repo.. just a PoC)

dbachelder#1