New issue 417: Allow negative prices
https://bitbucket.org/blais/beancount/issues/417/allow-negative-prices
Javier:
Could negative commodity prices be allowed? Some financial assets/contracts, like swaps, may have a negative entry price.
For instance, allow negative prices for the following asset-class
```
2020-01-01 commodity SWAP-T10
asset-class: "swap"
; Should not throw any warning
2020-01-01 * "Enter swap"
Assets:Broker:SWAP-T10 1 SWAP-T10 {-1234.56 USD,
2020-01-01} @ -1234.56 USD
Assets:Broker:USD 1234.56 USD
```