This change adds Affiliation parameter support to ListCollaborators
method by changing its ListOptions argument to a newly created
ListCollaboratorsOptions struct.

This is a neccessary breaking API change. The compiler will help catch
it, and updating code is easy. If the ListOptions argument was nil
before, it can stay nil after. Otherwise, you should change this:

	opt := &github.ListOptions{Page: 2}

To this:

	opt := &github.ListCollaboratorsOptions{
		ListOptions: github.ListOptions{Page: 2},
	}

GitHub API docs: https://developer.github.com/v3/repos/collaborators/#list-collaborators.

Fixes #647.