Added onVal, andOnVal, orOnVal to JoinClause

[wubzz]

Relates to #2707 - See discussion

Allows join using parameters instead of having to rely on Raw.

Before

this.on('p.id', '=', 'price.post_id').andOn('price.meta_key', knex.raw('"_regular_price"'))

After

this.on('p.id', '=', 'price.post_id').andOnVal('price.meta_key', '_regular_price')

[wubzz]

Reopening, GitHub isn't updating with commits..