Add option to set referrer/User Agent to possibly avoid 429 errors

[carstenhag]

As explained/documented at https://help.openstreetmap.org/questions/55828/overpass-returns-429-every-time-for-some-queries-but-not-for-others, it would be good for Overpass API consumers to include a referrer or an User Agent in order to not get blocked.

Also, with a referrer or some information about the client, the server maintainer could contact the client in order to solve the issue perhaps.

In the future, the Overpass API server could tell us why and when we should try again: drolbr/Overpass-API#351

[carstenhag]

As far as I understand, we should change/add properties to reqOptions.

https://github.com/perliedman/query-overpass/blob/master/index.js#L27