Add DE VVS Alerts translator

[hbruch]

The German VVS (Verkehrs- and Tarifverbund Stuttgart, https://vvs.de) publishes a GTFS-RT Alerts feeds, which has various issues:

1. stop_id ids are DHID parent station ids (though the parent stations are not contained in the corresponding GTFS feed)
2. Route-Ids don't match (though they are mostly mappable by a simple text replacement)
3. no efffect/cause are defined
4. descriptions are not plain text but html fragments.
5. priorities are not used

This PR currently addresses issues 1.-4.

stop and route ids, which can't be matched to the referenced GTFS stops/routes are logged as warning.
Note:

• currently reported warnings are caused by issues VVS: in GTFS-Feed fehlen temporär unbediente Stops GTFS-Issues#158 and VVS: GTFS-RT-Alerts enthält Alerts zu zukünftigen Linien GTFS-Issues#159.
• filling effect and cause is currently done by a simplistic text matching and will require further fine tuning.
• descriptions stripped from html may be hard to read and still to verbose. In addition, the often duplicate information already given in the header