Add add_select query

[josephmancuso]

A really cool feature of other orms is the ability to add a select query in place of the column name so you can select based on a relationship or based on another query.

An example of this type of query is:

select code, (select count(*) from order_skus where order_skus.order_id = orders.order_id) as sku_count from orders

This would select the order code plus the number of skus the order has and return the code and sku_count.

In the ORM this should look something like:

User.where('active', 1).add_select({'sku_count', lambda query: (
     query.where_column('order_skus.order_id', "orders.order_id").count('*')
)})