Allow setting of a "label" property for networks

I sometimes have different "identities" on the same network, meaning grouping by server hostname isn't enough. Chomp is currently making a 'temporary' label with which to group appropriate buffers: http://imgur.com/LFaFqmr

It would be nice to be able to have a user-defined "label" on a network. (I tend to use stuff like "Freenode (carrot)" or "ImaginaryNet (carrot-possel)" but people might prefer their own naming scheme).