Query Builder MERGE aka UPSERT

[willfarrell]

The ability to uniformly apply and UPSERT to multiple languages.

Great summary of the different SQL implementations: https://en.wikipedia.org/wiki/Merge_(SQL)

Possible Syntax: knex().insert({...}).upsert([keys]).update({...})

[rhys-vdw]

I'd like to see a discussion of the API, but upsert support would be great.

[rhys-vdw]

Duplicate of #54