support for commodities fx

[jbrodriguez]

this lib (and specifically the ledger-go tool) fails on this ledger file

;; Opening balances
2018/01/01 opening balances
  assets:savings:bank    $1234.56
  assets:crypto:bitcoin      BTC0.789
  equity:opening balances

the reason is there are 2 commodities and this line makes it fail

https://github.com/abourget/ledger/blob/master/journal/transaction.go#L47

func (tx *Transaction) ImplicitAmount() *Amount {
	var amount *Amount
	for _, n := range tx.Node.Postings {
		if n.Amount != nil {
			a := nodeToAmount(n.Amount)
			if amount != nil {
				if amount.Commodity != a.Commodity {
					return nil
				}
				amount.Quantity.Add(amount.Quantity, a.Quantity)
			} else {
				amount = a
			}
		}
	}

	amount.Quantity.Neg(amount.Quantity)
	return amount
}

is this a correct assessment or am i missing something ?