add new api selfplay example

[akashsara]

Adding in a version of selfplay that's compatible with the new gym API. I've not used any actual RL libraries in (since I wanted to put a general purpose example), similar to the older example but adding in library code should be trivial.

Relevant to #314, #317

[hsahovic]

Hey @akashsara - I missed the notification for this. Thanks for putting the time in opening this PR! I'll take a deeper look at it sometime next week :)