[Feature Request] Allow page size option (per_page) in listPullRequestFiles call

The listPullRequests call allows an optional second parameter for options that include setting the page size for for a request. GitHub allows `per_page` to also be sent along for the call wrapped by `listPullRequestFiles` but this library doesn't allow options to be passed.

I'm happy to submit a PR but want to know:
1. If you're cool with this
2. How you'd like me to add this functionality without breaking any existing uses of the library that rely on the second parameter being a callback. implement a `listPullRequestFilesWithOptions`?