[Suggestion] support appimage update

[ImpleLee]

AppImage may contain update information, which can be used to update the appimage with a single command:

appimageupdate /path/to/appimage

For the mechanism to work, we need to make the user supply a single string of update information, as specified in the spec, and then supply the string to the -u or --updateinformation option of appimagetool. Then the tool will generate an additional file called xxx.AppImage.zsync, which is required for delta update to work.

For your reference:
https://github.com/AppImage/AppImageSpec/blob/master/draft.md#update-information

Note:
The option is only working if zsyncmake is installed, which comes with the zsync package on ubuntu.