Support Upsert in postgres

[ZaneZ]

The statment of UPSERT in postgres:

INSERT INTO ... ON CONFLICT ON CONSTRAINT DO UPDATE ...

Currently in
https://github.com/Selvatico/go-mocket/blob/master/stmt.go#L16, command only takes in first word.

And for INSERT Case, there is only RowsAffected = 1 option if do not identify the error. https://github.com/Selvatico/go-mocket/blob/master/stmt.go#L86

Proposed solution, one of the two will work for postgres.

1. detect whether there is a update followed by insert

2. instead of checking command using first word, do a backward search of the key words lists.