5.13.0

Added support for converting from dBm to watt and back.

I also started adding support for "multi unit conversion', for example,
converting a time amount to hours, minutes and seconds.  It doesn't work
yet, but the syntax will be like this:

rpn 0.78 days [ hour minute second ] convert

I'm also considering changing the unit conversion to not permute every
unit with every other unit, but to do every conversion in 2 steps.
First convert the unit to a base unit for the measurement type (e.g.,
meters for length), and then convert it to second unit from there.

This means the unit conversion table won't have to have over 174,000
conversions, but will end up having 4N conversions for each unit type
instead of 2N^2 conversions.  That would also allow to do conversions
from dBm to any power unit, and not just watt, etc.  I've been thinking
about this for a while, and will think about it some more.