Send push notification about an incoming trolleybus

[roata]

The reason some people refresh very often (see #16) could be that they're impatiently checking the state of the incoming trolleybus.

We should give users an option to:
(a) say where they are, OR
(b) choose a station from the list

and indicate how many minutes in advance they want the bot to alert them about the incoming trolleybus.

This feature might become obsolete when https://github.com/roataway/web-ui is available, because people will simply observe it move in real-time on the map. Thus, implementing this is deferred until the situation with the web-UI is clearer.