Issue #176: Link one document to multiple accounts in one directive (blais/beancount)

[Yuchen Ying]

New issue 176: Link one document to multiple accounts in one directive
https://bitbucket.org/blais/beancount/issues/176/link-one-document-to-multiple-accounts-in

Yuchen Ying:

This is somewhat related to https://bitbucket.org/blais/beancount/issues/175/support-multiple-accounts-in-one-balance but I figure it can be used for other cases.

If issue 175 is implemented, a followup feature request would be able to specify one statement file for a balance directive with multiple accounts. E.g. my bank issues one statement for all the CD/checking/saving accounts. Something like this would be great:

```
2017-01-01 balance
Assets:Cash:CD01 xx USD
Assets:Cash:CD02 xx USD
Assets:Cash:Checking xx USD
document: "path/to/2017-01-01.statement.pdf"
```

Which should be equivalent to
```
2017-01-01 balance Assets:Cash:CD01 xx USD
2017-01-01 balance Assets:Cash:CD02 xx USD
2017-01-01 balance Assets:Cash:CD03 xx USD
2017-01-01 document Assets:Cash:CD01 "path/to/2017-01-01.statement.pdf"
2017-01-01 document Assets:Cash:CD02 "path/to/2017-01-01.statement.pdf"
2017-01-01 document Assets:Cash:CD03 "path/to/2017-01-01.statement.pdf"
```