Return the filename the video was actually saved to

This is useful both when downloading:
 * without specifying a filename, so it can be further processed.
 * with specifying a filename, as even in this case validation is performed (replacing / with -).

Of course, if you don't care about the filename you just ignore the return.
Merge pull request #12 from hugovk/patch-1

Return the filename the video was actually saved to