babeltrace(1): add help command

babeltrace(1): add help command

The new `help` command shows all the information of a specific plugin
or of a specific component class.

The syntax to get help for a specific plugin is:

    babeltrace help PLUGIN

and for a specific component class:

    babeltrace help (--source | --filter | --sink) PLUGIN.COMPCLS

The full help of the component class (bt_component_class_get_help())
is printed when available.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>