[Feature request] Implement `--appimage-html`

**User story:**

> *"As a user, I want to be able to access an HTML-based (and potentially other) documentation that an AppImage maintainer (or the developer of the respective app) has embedded into the image."*

I'd expect the HTML documentation of an application to be opened in my standard browser by simply running
*`myApp.AppImage --appimage-html`*
from the command line.

But maybe you guys can come up with an even more convenient method?

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^{(This feature request may need thorough discussion and preparation work first. A standard way to implement this should possibly be specified in the [AppImage spec draft](https://github.com/AppImage/AppImageSpec/blob/master/draft.md) document.}