[Feature Request] Add `--appimage-enum-binaries` to enumerate embedded binaries

I think there is a need to allow users to list the embedded binaries which can be started depending on what the name of *`${0}`* (or *`argv[0]`* for a compiled program) is when the AppImage is invoked.

Currently there are already a few AppImages on the "market" which support and make use of the feature to embed multiple binaries. But it is not easy to enumerate them (I *know* I could run *`appimagetool --list the.AppImage | grep bin/`*, but...)

The fact that this is even possible has escaped many developers' attention, even if they had already pondered to use AppImage for the distribution their own applications (see for example [this comment](https://github.com/mbunkus/mkvtoolnix/issues/2163#issuecomment-348687548)).

So the binaries inside an AppImage should be made more easily *discoverable*. One first step could be to implement a new parameter

--appimage-enum-binaries # Enumerate all startable binaries embedded in AppImage