Support granular diagnosis exclusion

[skeggse]

This adds a new excludeDiagnoses option (documentation pending) to Isemail.validate which specifies a set of diagnostic codes to exclude from validation. For example, consider the following diagnoses:

rfc5321TLD: 9,
rfc5321TLDNumeric: 10,
rfc5321QuotedString: 11,
rfc5321AddressLiteral: 12,

If you want to accept 11, and 12, but prohibit 10, you'd previously need to run isemail twice with two different errorLevels. That workaround doesn't scale well beyond a single gap, however. With the new excludeDiagnoses option, we can be more specific about what to allow through:

const options = {
    errorLevel: Isemail.diagnoses.rfc5321TLDNumeric,
    excludeDiagnoses: [
        Isemail.diagnoses.rfc5321QuotedString,
        Isemail.diagnoses.rfc5321AddressLiteral
    ]
};

Isemail.validate('joe@[127.0.0.1]', options) // => true
Isemail.validate('[email protected]', options) // => false
Isemail.validate('"joe"@hapijs.com', options) // => true

Fixes #180. Is also an imperfect solution to #162.

[skeggse]

Released in 3.2.0.