Issue #371: sellgains should handle or ignore losses (blais/beancount)

[Martin Michlmayr]

New issue 371: sellgains should handle or ignore losses
https://bitbucket.org/blais/beancount/issues/371/sellgains-should-handle-or-ignore-losses

Martin Michlmayr:

Enabling the sellgains plugin resulted an error message on all transactions in which I made a loss rather than a gain.

imho sellgains should either ignore transactions where the cost is higher than the price or invert its logic.

```
/home/tbm/personal/finance/d:13: Invalid price vs. proceeds/gains: (90.00 EUR) vs. (100.00 EUR)
```

```
plugin "beancount.plugins.sellgains"

2000-01-01 open Assets:Invements
2000-01-01 open Assets:Cash
2000-01-01 open Expenses:Capital-loss
2000-01-01 open Equity:Opening-balance

2019-01-01 * "Opening balance"
Assets:Invements 1 FOO {100 EUR}
Equity:Opening-balance -100 EUR

2019-02-01 * "Sold FOO at loss"
Assets:Cash 90.00 EUR
Expenses:Capital-loss 10.00 EUR
Assets:Invements -1 FOO {100 EUR} @ 90.00 EUR
```