Extend commands with named arguments

[netromdk]

Currently you have to specify the channel and/or user IDs when posting and inviting, which can be found using channels.list and users.list. But it would be easier to do

chat.postmessage -cn general "text to send"

To which -cn|--channel-name is needed.

This functionality requires to get the list of channels and match their names, something that channels.list already does so either abstract it or do directly if simple enough.