Improved the behavior of -X

[John Wiegley]

Up until now, -X support two forms:

-X '$' Value everything in dollars, else don't try
-X '$,EUR' Value everything in dollars, else Euros

This behavior has now been refined. If you say -X '$,EUR', Ledger will no
longer revalue EUR amounts into $. If something is already EUR, it will now
stay EUR, because EUR is in the -X list. Previously, Ledger would try to
value everything in dollars, even though you'd also asked for EUR.

To get the old behavior for any commodity in the -X list, suffix it with a !.
Thus, -X '$!,EUR' now means what -X '$,EUR' *used* to mean: Try to value
everything in dollars as much as you can; failing that, use EUR; failing that,
don't revalue.

This resolves feature request #231.

John