Issue #236: CONVERT() should allow date as optional parameter (blais/beancount)

[Martin Michlmayr]

New issue 236: CONVERT() should allow date as optional parameter
https://bitbucket.org/blais/beancount/issues/236/convert-should-allow-date-as-optional

Martin Michlmayr:

I want to use CONVERT() to convert something to its historical cost (i.e. exchange rate on date of transaction, without later revaluation).

I don't think this is currently possible with beancount. Maybe CONVERT() can be extended to allow an optional DATE.

Example:

```
2014-01-01 open Expenses:Test
2014-01-01 open Assets:Cash

2014-01-09 * "Test" "Test 1"
Expenses:Test 10.00 EUR @ 0.8283 GBP
Assets:Cash -8.28 GBP

2014-06-10 * "Test" "Test 2"
Expenses:Test 10.00 EUR @ 0.8106 GBP
Assets:Cash -8.11 GBP

2014-07-30 * "Test" "Test 3"
Expenses:Test 10.00 EUR @ 0.7914 GBP
Assets:Cash -7.91 GBP

2014-09-11 price EUR 0.8011 GBP

2014-12-31 price EUR 0.7825 GBP
```

```
select date, position, convert(position, "GBP"), balance where account ~ 'Expenses'
date position convert_po balance
---------- --------- ---------- ---------
2014-01-09 10.00 EUR 7.8250 GBP 10.00 EUR
2014-06-10 10.00 EUR 7.8250 GBP 20.00 EUR
2014-07-30 10.00 EUR 7.8250 GBP 30.00 EUR
```

It uses the latest exchange rate (0.7825) for all transactions.