Allow polling of a single address

[justusranvier]

In a setup where there are multiple addresses in a Bitmessage node, I'd like to be able to download only the messages that have been sent to a single address instead of all the addresses.

Perhaps it could work that if the username passed via POP matches an address belonging to the node you only get that address's messages, and if the username doesn't match you get all messages (current behavior).