Add allowUnicode option, default to true

[WesTyler]

We should include an option to "disable" international character recognition, per discussion in hapijs/joi#931 .

Default behavior should be to allow international, since that is the more recent RFC.

[WesTyler]

I arbitrarily called it allowUnicode just to have enough info for an issue. Defer to @skeggse on implementation details.

[skeggse]

I'm working on a rewrite that moves this module closer to being an email address parser with validation as added functionality. As opposed to using a massive state machine, it'll implement the parser as a recursive descent parser. It'll make the code way easier to work with, but in the meantime, I'll try to write a patch that adds this flag.

[skeggse]

c640d2e, landed in v3.1.0.