Generic support of extra files

[misli]

Hi,
I need to work manage GTFS feeds containing additional file (stop_external_ids.txt). I see that there is already an issue (#63) regarding other non-standard file.
I suggest to add generic support for extra files: define new model ExtraFile with columns feed, filename, and extra_data, which would handle any extra data. List of extra filenames would be defined in Feed.meta['extra_files'] and list of columns in Feed.meta['extra_files'] as usual.
I'll provide PR, but I want to discuss it first.
Do you have any objections or suggestions? Thank You.

[jwhitlock]

I think that would be an interesting way to take care of the issue. Can you find one or more feeds with extra files (preferably short ones) that we can add to the test suite?