Add a threshold below which replay gain is pointless

[bebelalu55]

First of all, thank you for providing this nice utility.
It would be nice to be able to specify a threshold below which writing the replay gain tags are meaningless.
I would myself consider that an adjustment of 1 or even 2dB as a limited effect.
I have to achieve this by letting rsgain do the job and then undoing it if the gain value is below my defined threshold