configurable ascii-only mode for identifiers

[slingamn]

Unchangeable via rehash, requires ascii for nicks/channels/accounts.

[slingamn]

Three options:

1. Status quo, with PRECIS casefolding and skeletons
2. Strict, ASCII-only
3. Permissive, allowing emoji

[DanielOaks]

It's worth keeping in mind that because emojis can't be folded effectively, a permissive mode that allows them inside identifiers is unsafe (you will be able to spoof the identifiers of people using emoji in their identifiers, afaik there's no way to do it safely right now). We don't want to encourage using that mode, so I might suggest a name that notes the danger a bit more and/or warning messages when that mode is enabled.