Feature/support cancellation token

[TheConstructor]

This builds on #196 and adds optional support for a CancellationToken.

I am open for redesigns and suggestions how to test cancellation.

[natemcmaster]

@TheConstructor I'd like to hold off on this until after 2.3 is done. I'd like to do cancellation tokens in a future version, but want to take the idea you have proposed even further so we can address #111. I don't want to rush anything in just before finishing 2.3. Let's revisit in January.

[TheConstructor]

@natemcmaster totally agreed that this is to big to go into 2.3.0 now. I mostly felt bad rendering the CancellationToken useless in #196 and wanted to assure we revisit it in the future.

[natemcmaster]

This PR has lots of potential and I like the direction you're headed. The library doesn't support cancellation token anywhere else yet (#153), and I think it's worth doing this cleanly and all the way through the stack instead of piecemeal. I'm definitely interested in doing this for 3.0.

Closing for now since I think this deserves more time and more work.