Implement beginning balances

[keithrbennett]

Currently, the way to enter beginning balances (e.g. of a checking account) is to record a General Journal entry with the balances, and date that entry the first day of the period (usually Jan. 1). This is suboptimal because it fails to differentiate between a balance preceding the period from a transaction during the first day of that period.

We need a way to record beginning balances that can't be confused with ongoing transactions. The natural place for this is probably the chart of accounts. However, we'd want to be careful to code the parsing in such a way that it will still be compatible with earlier journals that don't have these balances. And anyway, we'd probably want the beginning balance to be optional, since most accounts are income or expense accounts and will have a beginning balance of zero (a great default value).