First import of extras/ with lttng bash completion

Creation of the extras/ directory. Will mostly contains third part
scripts or tools not core to the project.

Adds the lttng bash completion script which can be copied to
/etc/bash_completion.d/lttng for first and second level command auto
completion.

Thanks to Simon Marchi for this contributions!

Signed-off-by: Simon Marchi <simon.marchi@polymtl.ca>
Signed-off-by: David Goulet <dgoulet@efficios.com>