At the moment, the heuristic used to determine connection targets is quite
simple: $station is a target iff there were at least two trips in the past
6 weeks with checkin at the current destination station and checkout at
$station.

Todo / enhanhement options:

* Consider current origin station as well, i.e.: $station is a target
  iff there were at least two trips in the past 6 weeks which are preceded by
  a trip from the current origin station and (same as above)
* Make connection links clickable once the current journey has ended.
  Connections need to be added to _checked_out as well for this to work.

This eliminates a large portion of main-thread work on page load

Closes #12

closes #10