When checking which packages changed, ignore commits with "[skip ci]"

[giltayar]

There is a convention in CI where if you write "[skip ci]" in the commit, then the CI shouldn't run because it's a commit that doesn't really need a build. Unfortunately, that won't work with Bilt, because the next build will look at this commit and say that some packages need change.

So Bilt should ignore commits with "[skip ci]" in their commit. Obviously, this string needs to be configurable via .biltrc.