Support phraseto_tsquery (postgresql 9.6)

[creichert]

Postgresql 9.6 adds support for "Full-text search can now search for phrases (multiple adjacent words)":

phraseto_tsquery('english', 'a multiple word phrase');

I would like to get this function added to esqueleto-textsearch.

FWIW, I don't mind adding this function myself but made an issue so we can document the patch + getting a new release on hackage.

[albertov]

A PR for would be most welcomed! I believe the implementation would be very similar to what plainto_tsquery does.

I could also release it to Hackage give that there is a need for it.

Thanks!