Allow magic commit message part to cause build to be skipped

[joshpencheon]

Commits tagged with variants of [ci skip] will be skipped over.

@timgentry - I have a niggling feeling you didn't want this feature, so deferring to you.

cc: @miles-smith

[timgentry]

Sorry, but I don't want this feature as it stands. Analysis of who did what (test coverage, failures, smelly code, etc) will generally depend on every commit being tested, which is also the main reason why we wrote RakeCI, rather than using Jenkins or something else.

I would prefer that we are smarter about when we skip. For example, we could look for a .rakecignore file within the root of a project and ignore those files. This would allow us to skip precise files, eg. user lists or code_safety.yml.

[timgentry]

I've added my requirements as #66.