Add Underpaid Status option

[astupidmoose]

Underpaid invoices are currently treated as "Expired". However, this sometimes causes confusion for the customers if they have used a wallet which takes fees from the total sent (Coinbase... ).

These customers will not realize they have underpaid and the shop owner does not realize a partial payment has come in.

Adding the ability to assign a custom status to this status will allow shop owners to be more proactive in their dealing with these customers.

[astupidmoose]

Closing this as its already been added. "Expired with partial payment"