abort builds with [skip ci]/etc

Even though CircleCI supposedly supports `[skip ci]`, I still usually see builds go through when I add that; not sure what I'm doing wrong, if they dropped it, or if something is misconfigured on our end. But we could work around that by changing the build scripts, or maybe `ff_ci_pr_build.py`, to check the commit message and abort if it sees `[skip ci]`.

Also, since CircleCI doesn't support `[skip circle]` and don't seem to want to (https://github.com/conda-forge/conda-forge.github.io/issues/45#issuecomment-198060065), we could more-or-less implement that here. Same for `[skip travis]` (https://github.com/travis-ci/travis-ci/issues/5032). Or we could implement `[skip mac]` or something to still work when we use CircleCI for Mac builds also, or similar.

Drawback: it would prevent you from manually running builds of commits that shouldn't actually have been skipped, unless we add something to do that.