Add ability to cancel try/merge runs

This is necessarily dependent on the CI service being used. It would be nice to not hardcode Circle CI-specific logic into the merge bot, so it would be great if we could design some way to "plug in" CI-specific cancellation logic.

Work started at: https://github.com/LeapYear/merge-bot/commit/2dfc2ab5156dddff65a551f5294bc1363938bb08. Feel free to use or not use.

Start by reviewing commit 2dfc2ab5156dddff65a551f5294bc1363938bb08 and the existing merge-bot CI integration. Define how CI-specific cancellation logic can be plugged in without hardcoding CircleCI behavior; done means try/merge runs can be cancelled through that provider boundary.

Written by the indexing model from the issue text.