Stashing specific files

I have sometimes wished for the ability to stash specific files, and I just had a user request for this as well.

There is the GIT_STASH_KEEP_INDEX option, which is one way to achieve this. Even better, though, would be the ability to pass either a list of files or a file filter function. I think the latter might be better since it's more powerful.

I'd probably be able to tackle this myself, so any feedback on appropriate API design and such would be great.