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']

fdroidserver/common.py

@@ -59,6 +59,7 @@ default_config = {
     'ant': "ant",
     'mvn3': "mvn",
     'gradle': 'gradle',
+    'accepted_formats': ['txt', 'yaml'],
     'sync_from_local_copy_dir': False,
     'per_app_repos': False,
     'make_current_version_link': True,