Add generalized support for Media Internet files

The Migrate Extras module currently has specific support for migrating YouTube videos, and there's a patch at #1820012: Flickr file interface for Flickr support.

However, the point of the Media Internet module is that all of these things can and should be handled in abstract way. So rather than writing specific handlers to support each of them, here's a simple patch that should support any/all of them at once. It also saves the file using the standard API for doing so, which means it will automatically take advantage of any processing that normally happens when a file of that particular type is saved using Media Internet.