start on u verb progressive

@hermanschaaf maybe you can help me here - the way to get progressive for a Ru and U verb is exactly the same: for progressive positive you take the te form and add いる, for progressive negative you take the te form and add いない, etc. But of course, getting the te form differs for Ru and U verbs. Can you think of a way I can share the `Progressive*` functions for both Ru and U verbs so that I don't have to duplicate them all? I guess ideally they would be defined as a method on `*Verb` instead of `*RuVerb` and `*UVerb`, but then the `TeForm()` function needs to know what type of verb it is.