Add "DISTINCT ON" clause for SELECT

[ikozinov]

I couldn't use the extensibility features for sqlbuilder, so I added "distinct on" clause support for PostgreSQL right in sqlbuilder

[huandu]

Thanks for your contribution.

DISTINCT ON clause is a PostgreSQL-specific syntax. I don't think it's suitable to add it to SelectBuilder as a general API which should work for both MySQL and PostgreSQL.

However, it's reasonable to enable package users to build this clause with SelectBuilder. I'm think of a design with which we cna add any SQL clause to a builder freely.

A rough design looks like following.

// New API SQL() enables us to add any SQL after any clause generated by a builder.
// In this case, as SelectBuilder always starts with SELECT,
// the clause added by SQL() is inserted after SELECT.
sb.SQL("DISTINCT ON (col1, col2)").Select("col3", "col4")

Any feedback? You suggestion is highly appereciated.