add initial help screen + keybinding -> command parsing

still needs some more work, i.e. to show commands in help instead of parsed data
structures, but it's a start.

fixes #117
fixes #121

README.md

@@ -60,6 +60,7 @@ the `portaudio_backend` feature:
 These keybindings are hardcoded for now. In the future it may be desirable to
 have them configurable.
 
+* `?` show help screen
 * Navigate through the screens using the F-keys:
   * `F1`: Queue
     * `c` clears the entire queue