Add command to switch rooms

[aurbano]

Proposing /next to switch to a different room (if available)

If a room is available it will switch, clearing the local history and going into the new one, along with a system message confirming the switch (that should appear after the new history is loaded, so it shows up at the bottom)

If no other room is available the server will reply with a system message saying that there are no other rooms available.