Implement ebirdsubregionlist, to cover ref/region/list/ API endpoint.

[dbradnum]

Description

Tidying up another snippet of code I had locally, to implement another missing API endpoint - this time to find eBird subregions within a given region.

I hope this is useful and doesn't create much additional work while you're preparing for a new release. However, let me know if you spot any problem or would prefer not to merge this in at the moment.

Example

ebirdsubregionlist("country", "world")
ebirdsubregionlist("subnational1", "US")
ebirdsubregionlist("subnational2", "US-NY")

[sebpardo]

Thanks for these functions @dbradnum, they look great! I haven't used PR review before so I decided to give it a go, hopefully it makes it easy for you to consider, and when warranted incorporate, my suggestions.

[dbradnum]

And thanks for these comments, too. I think all of these are sorted now.