Expose Spendable Balance

[ge33ek]

As the title suggest, expose the spendable value via API so that it can be used for calculations.

[damian-w]

+1 on this request... 'Spendable' is one of the great features of Up, and it'd be even better to expose this in the API.

[ajgold]

'Spendable' is linked to pay cycle and Upcoming transactions. Please see related request #118

[johnmee]

Not the same, but not entirely unrelated, to #78

[markbrown4]

We're unlikely to expose running balances outside of statements.
This feature would be exposing the spendable balance on an account at the time of request only.