* Add split one token into several (resolves#2838)
* Improve error message for token splitting
* Make retokenizer.split() tests use a Token object
Change retokenizer.split() to use a Token object, instead of an index.
* Pass Token into retokenize.split()
Tweak retokenize.split() API so that we pass the `Token` object, not the index.
* Fix token.idx in retokenize.split()
* Test that token.idx is correct after split
* Fix token.idx for split tokens
* Fix retokenize.split()
* Fix retokenize.split
* Fix retokenize.split() test