Adds support for `transition-behavior` property

[bes-internal]

[cjohansen]

Any chance this could get merged? I just spent quite a bit of time tracking down weird behavior that turned out to be clean-css removing the transition property when using allow-discrete. This would help a lot 👍

[magnars]

As a point of information in favor of merging this PR: I switched to this branch in one of my projects and ran its test suite, and everything still worked, including the new transition-behavior test that was failing prior to this fix.