Added a new function to TraversableExtras - split.

Split takes a predicate and returns an Iterable of Traversables of all
the elements that satisfy the predicate, as split by all the elements
that don't satisfy the predicate. Concretely `(1 to 20).toSeq.split(_ % 2 ==
0)` will give a list seqs that contain 2, 4, 6, 8, etc.

Useful for partitioning OWPL-style files into seqs of sentences.