Filter external data results

[brocksm]

As far as I can tell, Dakboard does not currently allow the user to filter results from external API calls (in the case the API does not accept URL parameters to filter the results in the call itself). Because I don't know what index of the API results I will find values relevant to me, I can't use the manual selection option that Dakboard currently provides.

I am trying to set up a block that contains the arrival times for the next few trains arriving at my local commuter rail station. I would like to be able to filter the results doing something like stop_id = ${station_name} and arrival_time = ${next 30 mins} (relative values to the current time would be very useful here).

I think this FR would also cover the use case detailed in #1828, but in a more generalizable way.