Support PostgreSQL ON CONFLICT

[kipply]

https://www.postgresql.org/docs/current/sql-insert.html

I'm looking to write a query like;

	INSERT INTO table ("col", "col2", "col3") 
       VALUES ($1, $2, $3), 
              ($2, $1, NULL) 
ON CONFLICT("col") DO NOTHING RETURNING col;

It seems `InsertReturning` seems close but is conflict handling supported anywhere? Alternatively, are there any viable workarounds to achieve the same effect or effectively write a raw insert query?

[VojtechVitek]

See https://github.com/upper/db/pull/298 and https://github.com/upper/db/pull/318.