Provider-independent environment variables

[liclac]

Right now, there are two sets of train data sources (ICE, Eurostar), and they both output incompatible sets of data, meaning we also need multiple sets of widgets.

I think maybe along with AMBIENT_DE_ICE_SPEED and AMBIENT_EU_EUROSTAR_SPEED, we could add an AMBIENT_SPEED variable, and so on.

[liclac]

Thought: What if we just populated AMBIENT_SPEED, etc. and just made those up as we go along? It'd probably be the easiest way to do it, and we could add a section to the README listing all the different variables.