Crash when trying to collate pretokenized witnesses with space or punctuation about collatex

[tla]

I put this fix on a branch because it does change the API a little. Collation.add_witness now takes a dict structure that looks (coincidentally enough) like the JSON specification of a witness (whether pretokenized or not). I have added a method, Collation.add_plain_witness, that preserves the old behaviour, and I've changed the tests to use that instead. If you (@rhdekker) think the fix is fine, then you can merge it to master and close this.

[rhdekker]

Fix is fine, the responsibilities are better distributed over the classes in the patch. Merged into master.