Feature request: Add option to integrate metainfo.xml

[Daeraxa]

When building an appimage via a basic appimagecraft configuration, you get the following warning to the log:

[appimage/stderr] WARNING: AppStream upstream metadata is missing, please consider creating it
[appimage/stderr]          in usr/share/metainfo/myApp.appdata.xml

Proposal would to be another option like -e and -d that can handle the appdata file by providing a path e.g. -m "$PROJECT_ROOT"/assets/myApp.appdata.xml