Add support for lookalike value in audience schema.

[chetmancini]

Resolves issue #441 .

Adds support for lookalike values in schemas as documented here, without the end developer needing to prehash values: https://developers.facebook.com/docs/marketing-api/value-based-lookalike-audience-targeting

Pulled out logic into multiple class methods to replicate the normalize fields pattern, but happy to use another stylistic approach.

Thanks!