Please add support for custom build commands

[AbhyanshAc]

Please add support for custom build commands so that, let's say, before release, I need to remove or add os OS-specific assets, or let's say I want to publish to snapstore and I need it to automate publishing, so please add it.

[eximius313]

Really important feature!
It would work great in tandem with dart_pubspec_licenses package (generate required OS licenses and then, thanks to #305 include them in the generated distribution package)

BTW: this issue looks like a superset of #272