Add completion character after autocompleting a nick

[trevarj]

This was reported in IRC, I am just relaying it for visibility.

Similar to hexchat or irssi, it would be useful for some people to have a character placed after a nick is autocompleted, such as a colon or comma (i.e nick: reply message or nick, reply message). This should be configurable and only occur when the nick is at the beginning of the line (I think).