Make get-buildings more robust

[cholmes]

The get-buildings command seems to work decently well, but it does very little in the way of catching all the things a user might input things wrong. It'd be good to test out common ways that it wouldn't work right and give better warnings, etc.

It also appears to hang if you don't supply the file, I think since it's expecting stdin. I definitely want to keep the ability to do stdin, but ideally can warn users. In planet CLI we had '-' mean 'read from stdin' so that could be a good pattern to follow.

[cholmes]

One thing to test in particular is GeoJSON input that is not valid, is a point or line, or has more than one polygon. We should get to some nice default behaviors, and warn users what's happening (like auto-buffer points or lines, automatically select the first polygon (and/or give an option to union all polygons).

[felix-schott]

Hi Chris,
Thanks for developing this tool. I'm happy to work on GeoJSON validation! Expect a PR in the next couple days :)
Felix