Issue #351: Allow a balance column by referencing another column, sum up position converted at original rate (blais/beancount)

[Martin Michlmayr]

New issue 351: Allow a balance column by referencing another column, sum up position converted at original rate
https://bitbucket.org/blais/beancount/issues/351/allow-a-balance-column-by-referencing

Martin Michlmayr:

I [posted this to the beancount list](https://groups.google.com/forum/#!topic/beancount/SGDBejEtd0I) and Martin said this isn't supported at the moment. Opening a ticket.

I have expenses in one currency (EUR) and want to convert all of them to another (USD) **on the day** of the transaction.

This gives the correct result because I pass "date" to CONVERT():

```
beancount> SELECT sum(convert(position, "USD", date)) WHERE account ~ '^Assets:A';
sum_conver
----------
107.50 USD
```

But I'd like to see a "register" type view with each individual transaction and the total.


Test case:

```
plugin "beancount.plugins.implicit_prices"

2000-01-01 open Assets:A
2000-01-01 open Assets:B

2018-12-05 * "Test 1"
Assets:A 20 EUR @ 1.10 USD
Assets:B

2018-12-06 * "Test 2"
Assets:A 45 EUR {1.14 USD} @ 1.14 USD
Assets:B

2018-12-07 * "Test 3"
Assets:A 30 EUR {1.14 USD}
Assets:B
```