Metrics: Allow to search for relays per continent

During the last relay meetings it was pointed out that one goal is to increase the relay diversity around the world so I went to check for relays on other continents.

• Currently the metrics interface only allows to see relays for each country but not for continents.
• Also it would be nice to be able to query for more than one country at once. At the moment only the first is shown: https://metrics.torproject.org/rs.html#search/country:al%20country:dz

(if you prefer i can file a separate issue for the second one)