This replaces the implementation for custom configparser option
references in the diff-file option. Moves detection and handling for
the custom syntax into a dedicated submodule. Uses a regular expression
to preserve text before and after the reference.

Restore original behavior where a diff file is only written if
the 'mirror.diff-file' config option is set to a non-empty value.

Change the Storage base class and the storage plugin loading functions
to require a ConfigParser instance so the BandersnatchConfig singleton
is not accessed.

Change the BandersnatchMirror class to require a Storage instance
instead of a storage backend name since the mirror subcommand already
loads a storage plugin instance.

The diff in the application code is relatively simple, but both changes
caused cascading/rippling updates to the unit tests that will require a
regrettable amount of tedious review. It would be nice to find a way to
split up this commit.

Change the Filter base class and filter loader to require an external
ConfigParser instance be passed in.

Change the Mirror/BandersnatchMirror classes to require an external
instance of LoadedFilters.

op/ed: Remember all that nonsense you told yourself about removing a
singleton making the tests simpler? Well so far that is not the case.