Help user determine region by lookup against the eBird API

[synrg]

Rather than have the user carry around eBird region codes in their head, it would be good to provide a way that they can lookup regions using the API.

• Given that an exhaustive list of regions is not very useful, provide commands to browse down to a reasonable subset of regions to choose from.
• Also, assuming the regions don't change very often, cache the result for a fairly long time (a day, perhaps) to avoid unnecessary API calls.

e.g.

[p]ebird regions <region>

Where the help would describe some common region values, and [p]ebird regions would list regions that are direct descendants of that region (but not recurse any further), paging output if necessary.