Allow ignoring files with paketkoll check

[VorpalBlade]

Broken out from issue #157.

Only check-unexpected allows for passing ignores currently. It would be nice to have that for check too, though the implementation is quite different, so it would be a separate implementation.

It should not be done as a filter at the end, it would be more efficient to simply not check files at all. Look into if the API of the ignore crate that we currently use is suitable or if we need to build our own regex-set instead. We shouldn't need to take on a new dependency for this.

cc @mbideau (original reporter)