Add service change/interruption information

[jpreardon]

This PR (optionally) adds service information from the subway alerts JSON feed.

• If "SERVICE_ALERTS = True" service alert information as HTML (when available) will be added to the stations. This includes stop based alerts and route based alerts for all routes servicing the station.
• This should be backwards compatible. Service alerts are only added if the configuration exists.

Open questions:

• Would it be better to add route based alerts to the individual train times?
• Should the alerts always be plain text rather than HTML. I've noticed that the plain text can include things like "/n", so I went with HTML. But, HTML isn't available for elevator outages, so it falls back to plain text in those cases.

Closes #1