Improve nick completion
Improve nick completion

Previously, weechat would not complete @-prefixed nicks, fix that, and
make it so that if you complete on an already-full nick, wee-slack
adds a prefixing @, leading to a slack highlight.

scharfie/wee-slack,trygveaa/wee-slack,rawdigits/wee-slack,megalithic/wee-slack,wee-slack/wee-slack