Add station and line as optional variables

I'm getting my ducks in a row towards possibly maybe making a Bicycle Baromter for the Pebble, and I thought it might be useful to have the station and line configurable via the query string, i.e.

http://0.0.0.0:5000/api?station=Euston&line=Northern

Will return the status for the Northern Line and Euston.

It defaults to Brixton and Victoria, so existing functionality remains unbroken :smile: