babeltrace-cfg: add the --path option

babeltrace-cfg: add the --path option

--path=PATH is a shortcut for --params=path=PATH. The main use
case is to use tilde expansion when using babeltrace from a
shell:

    -P ~/my-traces/...

is way more convenient than:

    -p path=\"$HOME/my-traces/...\"

In Bash and Zsh, tilde expansion does not happen when the tilde
character is not the first of its word.

--plugin-path looses its short option (-P). I believe --path will
be used more than --plugin-path.

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