Add discover-my-major

A better interface for getting information about major mode features. It can
complement the built-in "C-h m" (describe-mode): "C-h m" can be thought of
a full mode help, while discover-my-major a quick mode help, in the following
ways:

- Show key binding with description
- Cursor is automatically switched to discover-my-major help buffer, and to quit
the help buffer, simply press 'q'. In stock "C-h m", user has to manually switch,
scroll up/down to look for major mode commands. Finally, user closes the buffer
either by "C-x 0", or winner-mode undo (C-c <left>), which requires more key strokes.
- Display just enough information. In "C-h m", it displays every thing, from
major mode to minor modes, enabling minor modes and description of every modes.
- More polished user interface.

It's also better than smex-major-mode-commands, since smex-major-mode-commands
does not show key bindings and description.

core/prelude-mode.el

@@ -65,6 +65,7 @@
     (define-key map (kbd "C-c h") 'helm-prelude)
     (define-key map (kbd "C-c I") 'prelude-find-user-init-file)
     (define-key map (kbd "C-c S") 'prelude-find-shell-init-file)
+    (define-key map 'help-command (kbd "C-m") 'discover-my-major)
     ;; make some use of the Super key
     (define-key map [?\s-d] 'projectile-find-dir)
     (define-key map [?\s-p] 'projectile-switch-project)

core/prelude-packages.el

@@ -49,6 +49,7 @@
     anzu
     browse-kill-ring
     dash
+    discover-my-major
     diff-hl
     diminish
     easy-kill