Convert the decimal separator of numbers

It use two option, one declaring the what separator is used in csv, the
other in ledger. If both or none are set, no conversion is
needed. Otherwise a conversion is needed.

I wonder if this could be made cleared by defining a --convert-decimal-comma-to-decimal-dot and a --convert-decimal-dot-to-decimal-comma option. It seem more natural in icsv2ledger to use declarative option as I did, but I'm not completely sure of myself.