set "accepted formats" for metadata in config.py

For a bit repo like f-droid.org, it makes sense to standardize on a single
format for metadata files.  This adds support for enforcing a single data
format, or a reduced set of data formats.  So f-droid.org would run like
this if it changed to YAML:

  accepted_formats = ['txt', 'yaml']

Then once everything was converted to YAML, it could look like this:

  accepted_formats = ['yaml']