Respect `weechat.completion.nick_completer`

[BrendanTCC]

I may very well be the only user that's actually messed with the setting, so I can understand why Weechat-Android just defaults to : text but it'd be nice if there was at least an option in the app settings to pull weechat.completion.nick_completer from the Weechat instance and use that when tab-completing over : text.

I haven't looked closely at the relay spec, but I'm guessing it's possible since Glowing-Bear appears to do it.

Cheers.

edit: Clarified that I was referring to the relay spec 😅