Balance export in ledger format

[hrj]

This is useful to get previous years closing balances as opening balances for next year.

This will be equivalent to the equity report in ledger-cli

[hrj]

Idea: the closing of balances can be done as part of this export.

An example configuration could be:

exports += {
  type = "balance"
  closures += {
    from = [Income, Expense]
    to = [Capital:Profit]
  }
}

This will close the Income and Expense accounts completely and transfer their amount to the Capital:Profit account, and print the resulting balance statement.

If the closure option is useful in other reports too, it can be moved later to global scope in the config file.

One more option that can be considered is to "split the closure" or not. If split, then the the output will contain two transactions like this:

2013/3/31
  ; Balance before closure
  ...
  Income             4,000
  Expense           -1,000
  Capital:Profit    10,000

2013/3/31
  ; Closure
  Capital:Profit     3,000
  Income            -4,000
  Expense           +1,000          

Note: The date will need to be specified in the config file.

[hrj]

Missing from sheetalk's PR which I need to follow up:

• use trait AccountMatcher if possible.

More things TODO:

• Allow accounts to be filtered in export

[hrj]

Created follow up issues #95 and #96