Do not trim nicknames at ~

As seen in my Charybdis pull request atheme/charybdis#17 (which you fixed yourself, by the way)...

> First, it's 2013 and I'd be really surprised if anybody still cared
> about ISO-646 support these days.
>
> Second, ISO-646-FI, which the "scandinavian origins" probably refers
> to, did NOT have any letters in place of ~ or ^; it had the same ~ ^
> and so did all other Scandinavian ISO-646 charsets.  The only
> exception here is ISO-646-SE-C, which had Ü ü in those places, but it
> also had É é in place of @ `, so by this logic the code should
> disallow ` as well.
>
> Fuck that. If this patch hurts ISO-646 users, that's a feature.