Integrate gman government domain listing

[waldoj]

Use @benbalter's gman—or, rather, the list of domains—to optionally limit the acceptable domains. Or, at least, document how might one do that. We'll want to limit the @opendata site to just government domain names with the U.S. (most of which are not within the .gov TLD), so I figure I might as well make it easy for others who want to do the same with domains for particular states or countries.