[sqlgen] Insert: add support for column specification

[fsouza]

It should be possible to generate an insert statement only for given fields. Something like this:

type Person struct {
    Id int
    Name string
    Age int
}

p := Person{Name:"Francisco", Age:"23"}
sql, _ := sqlgen.Insert(p, "name", "age")

It should generate:

insert into person (name, age) values (?, ?)