Support multi-episode files

I think the easiest way to deal with this is to change the way episodes relate to a renaming run. There needs to be an iterable of episodes that the `FrontEnd` can loop through calling out to the correct methods for things like episode title and output formatting.

Something will need to keep track of this iterable, probably the `TvRenamr` instance that the `FrontEnd` can interrogate as necessary.

The regular expression method will need to change too - it should be the place where we decide on the number of episodes we're dealing with. Maybe just make it return a list of kwargs to build `Episode` from.