Change long form of save-image --ignore-app-descriptor

[DaneSlattery]

Solves #973

[SergioGasquez]

The change looks good, and it's something we definitely want to do, but this would be considered a breaking change that requires a major bump. We could support both ignore_app_descriptor and ignore-app-descriptor or just wait and merge this when working towards v5.

[SergioGasquez]

Can you modify this PR to support both ignore_app_descriptor and ignore-app-descriptor while adding a TODO that the underscore version should be removed for v5??

[DaneSlattery]

I've used visible_alias in the macro, which shows up like this in the help. It can also be a hidden alias if you prefer, to discourage future use

      --ignore-app-descriptor
          Skip checking whether the app descriptor is present in the image
          
          [aliases: --ignore_app_descriptor]

[SergioGasquez]

LGTM! A visible_alias should be fine, thanks!