Add option to set global attenuation (volume) value when exporting

[sound8]

Might be nice to have the option to set a global attenuation (volume) value when exporting.
This would allow users the ability to globally increase or decrease the exported volume value for Soundfonts.

Adding an option for this in the Command Line Export would be great, as there is no way to do this from command line currently. (The Command Line Export was an amazing addition to Polyphone so glad to see this was added, Thank You)

This could be a possible solution to volume issues when exporting SFZ or GrandOrgue instruments to Soundfont
#147 & #173
(If there is a standard volume db difference as I mentioned in post 147 then may be better to just add this compensation for SFZ / GrandOrgue to Soundfont converts automatically so users don't have to think about it at all, but having the ability to adjust to desired volume may also be a nice option as well.)

Hopefully there is some kind of solution available, as can't really use Polyphone / Command Line Export tool as a converter at the moment until there is one.

Thank you again for all the great work you do, what an amazing program you have created, and it keeps getting better.

[sound8]

Kind of strange seems to be no activity on this Github page?
Is this the official Github for Polyphone?
I made quite a few posts here and not a single response?
Was hoping this one out of all the posts would be considered to be added, if Polyphone is still being updated.
I have also updated the findings for issue 147 which was what lead to this request in the first place.