Avoid adding : when tab completing something that is not a nickname in the channel

[smakkerlak]

Current: typing out characters that is not an active nickname in the channel makes Halloy add ": ".

Wish: Nothing should happen if the characters at the caret are not an active user in the channel.

Optionally: configuration for how tab complete behaves. Adding : or other character(s) or just completing the nick. Could be simply a string of what to add after the nick, defaulting to ": ".