Add support for using bounding boxes in filenames to short-circuit computation

[mfogel]

New feature:

For the difference operation, add an additional optional command line flag -b / --bboxes. If the --bboxes flag is present, filenames of GeoJSON files should be scanned for stringified bounding boxes. If found, that bounding box should be compared with the subject's bounding box. If there is no overlap, then there is no need to read that file in off disk.

The format of the bounding box in the filename would be the same as is produced by geojson-cli-explode