add inverse-scale property

When decoding the input, the output lattice is scaled using the acoustic
scale parameter. When 'inverse-scale' is set to TRUE, the lattice is
re-scaled back (inverse the acoustic scale) before the hypothesis is
extracted.

This behavior is common when playing with different scales after the
decoding phase.

Signed-off-by: Amit Beka <amit.beka@gmail.com>