Provide DEFAULT on update capability

[Deiz]

Is your feature request related to a problem? Please describe.

Ideally, timestamps are database-managed.

If you have a struct like this:

type User struct {
    Name       string    `db:"name"`
    CreatedAt  time.Time `db:"created_at" goqu:"skipinsert,skipupdate"`
    ModifiedAt time.Time `db:"modified_at" goqu:"skipinsert"`
}

created_at is taken care of and is a write-once, update-never column, assuming the schema defaults it to now().

modified_at works well on insert, but thereafter either must be set to a zero-value and used with goqu:"defaultifempty" on update, or be set to time.Now() which is sub-optimal.

Describe the solution you'd like

I'd like to implement goqu:"defaultonupdate" in order to always set the column's value to DEFAULT on update - thoughts? I can't think of a circumstance where I'd want this aside from "last modified" timestamp values but that's something that most people care about, and which currently requires per-update boilerplate.

defaultonupdate would be lower-precedence than skipupdate, as well.

[Deiz]

@doug-martin thoughts?