Balance mode for accounts

[dvankley]

Add a configuration option for polled mode to set accounts as either "transactions" (the default, existing mode) or "balance."

The new balance mode would be used for accounts that either don't support the Plaid "transaction" product, or accounts that do ostensibly support the "transaction" product but never actually post any transactions.

For accounts using the new balance mode, instead of pulling transactions, the connector would poll the Plaid "current balance" endpoint and insert transactions into Firefly to match up the account balance with the value from Plaid.

[dvankley]

Closing this for now in favor of #68 since I think that will probably solve my actual issue. Will bring this back later if that doesn't sort it out.