Support Links pagination header in repos function.

GitHub uses the `Link` header to provide links to other "pages" of results. I've modified the `repos` function to really return all the results for a given user/org.

This should probably be some kind of an option for the functions in general, to pick whether to return 1 page of results or all the results. Pagination is annoying to do by hand, and can be pretty easily integrated into the Array-returning functions.