Handling unsaved buffers (especially with renaming)

While working on the rename feature, I've wondered if it was (easily) possible to make it work on unsaved buffers?
It's probably limited by Jedi only being able to handle saved files by default?
It might be possible to pass in any unsaved buffers as `Script` objects directly?

I've experimented with a new setting `rename_save_buffers`/`save_on_rename`, which would call `:update` on buffers where something was renamed.

What do you think?