Add support for named parameters

[ajzo90]

This PR adds support for named parameters. It tries to keeps old behaviour intact, but introduces some vague interpretation since it can mix named and non-named parameters. Possibly a better alternative is to check whether named or positional parameters are used and do strict validation.

stmt, _ := db.Prepare("SELECT $foo, $bar")
_ = stmt.QueryRow(sql.Named("foo", 1), sql.Named("bar", "baz"))`