Fix: allow specifying more than one input trace path

We need to fix this right away: leaving the OUTPUT cmd line argument
take away the possibility to specify multiple input trace paths, with is
a very common use-case when merging traces from various nodes.

Now, output can be changed by specifying -w, --output OUTPUT. This is a
rare use-case anyway, to allowing multiple input paths outweight the
need for cmd line arg compatibility, even at this stage.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Acked-by: Julien Desfossez <julien.desfossez@efficios.com>