Add a oneChangePerToken option

[ExplodingCabbage]

Resolves https://github.com/kpdecker/jsdiff/issues/148

To Do:

• [x] Docs
• Tests:
  • [x] At least 2 different diffFoo methods
  • [x] Test the case where the texts are identical
  • [x] .. and obviously also the case where they aren't identical