Add AppImage support

[io12]

AppImage is another useful format besides rpm and deb.

https://appimage.org/

[ndarilek]

I need this too and was thinking of attempting it.

Seems like the other implementations write their formats natively, but AppImage looks a bit more challenging as it's essentially a squashfs/ISO9660 image. Would an implementation that calls out to the official AppImage tool be accepted? I really don't want to implement the spec directly if I don't have to. :)

[mdsteele]

Yes, that would be a welcome contribution! It would be great to have support for this. (It's nice to not need to install additional tools to be able to use cargo-bundle, but support for AppImage that requires installing tools is definitely better than no support at all!)

[ndarilek]

Cool, I was just pointed to linuxdeploy as a good tool for automatically creating appimages. I don't know much about the format, but I'll try to generate packages on my own first, and will then see if there is any benefit to integrating the process with `cargo bundle`. At least the appimage tooling is almost entirely self-contained and pretty easy to install.