Add general maps and aliases

[cypherfox]

this provides a defined type to handle maps.

[mjhas]

very nice!

[andyleejordan]

Oh man, I really like this! I was just adding some virtual_alias_domains and a map to go with it today. Will use this instead.

[lingfish]

Forgive me -- but this won't work for alias maps, as postmap is for maps other than aliases... it needs to use postalias or newaliases?