When generating a new posting as a multiple of an existing posting,
support conversion to a different commodity. For example, postings in
hours can be used to generate postings in USD.
Automatic transactions generated from rewrite rules use the commodity,
amount style, and transaction price if the rewrite defines a commodity.
This allows period and comma to be used for decimal point and digit group
separator or vice versa, and also flexible digit groups. See
http://en.wikipedia.org/wiki/Decimal_separator . Digit group separators
are possibly not worth the trouble and might not stay.