Ctrl-D was not caught in a special case so it was interpreted as
a standard character. Handle it now the same way like EOF and
terminate the shell instance.
Introduce optional user shell_post_readline_hook, shell_pre_command_hook, shell_post_command_hook.
Enable with USEMODULE=shell_hooks.
Calls user implemented *_hook functions if defined.
If implementation does not exist, nothing happens.
The intent is to make profiling of the shell command timings easier.
Test provided in tests/shell with USEMODULE=shell_hooks.
