Track how much was spent from an expense per transaction.

[elliotcourant]

When a transaction is spent from an expense we need to keep track of how much was spent. For example; If a transaction is $10 and is spent from an expense that only has $5 allocated, then we cannot spend all $10 from it. We should thus keep track in a separate object, or as an additional field on the transaction itself for expenseAmount. This way if the expense a transaction is spent from changes we can easily revert the amount on an expense.

This solves an issue relating to #46

[elliotcourant]

For now I've added the field to the transaction object. I will add additional handling of this fields as I get expenses more built out.

[elliotcourant]

This is now implemented