Add support for help subcommands (C-h)

[dakrone]

This is a feature request for adding a discover commands for C-h. I know it already supports popping up a buffer with commands using ?, but discover support would be more visually pleasing.

[mickeynp]

I'm not too keen on overriding C-h as that is a fairly encompassing command that includes nested commands. Typing C-x C-h is a good example. My long-term goal is to update discover.el so it supports most of built-in Emacs - rendering the need for this feature redundant (I hope!)

If you have any particular areas of Emacs that warrants attention do feel free to say.

I have also updated the documentation and code to support third-party packages.

[dakrone]

Makes sense, thanks!