Filter (e.g., distance < X, altitude < Y)

[mikeage]

It would be very useful if we could add filters to the aircraft displayed. E.g., I'd like to only show planes within 10 NM, under 10k altitude.

I'm not sure what the best syntax would be; there is an existing hide: empty option, but I'm not sure what the cleanest way to be would be represent the filters that need a value, and whether it should default to and or or (and how to specify the opposite behavior), which is one reason I'm not submitting a PR. (the other two are poor JS skills and extreme laziness!)