Add `filterNot` to RichPipe

I found myself using `filter` with with negated predicates a lot recently which don't read as nicely as Scala's `filterNot`.

I thought that maybe more people would be happy about including `filterNot`, since it makes RichPipe "more scala collections-api complete" without really adding much maintenance cost.

Please comment if you have feelings against adding a new method to `RichPipe` :-)

Cheers and a happy :christmas_tree: !