Provide a parser interface and implementation for [fallback mirror lists](https://gitweb.torproject.org/tor.git/tree/src/or/fallback_dirs.inc) (cf. parent ticket). Naming suggestions: * new interface: `o.tp.metrics.descriptor.FallbackMirrors` with possible sub-interface `FallbackMirrors.Mirror` or shorter `Fallbacks` and `Fallbacks.Mirror` * package for implementation: `o.tp.metrics.descriptor.fallback`