Support EmailField

[gmacon]

(Imported from my local TODO file.)

Support EmailField. I have the comment "lookaround assertions".

[gmacon]

Hypothesis now has an emails strategy, but, before we can use it to support EmailField, it needs to support customization of thedomain part so that we can honor the domain_whitelist. That would also allow us to generate more valid examples with allow_ip_domain=True. Customizing the local part would allow us to generate more valid examples with allow_utf8_user=True.