Refine header regexp so that they do not overlap

Signed-off-by: Damien Cassou <damien.cassou@gmail.com>
Add imenu support

Signed-off-by: Damien Cassou <damien.cassou@gmail.com>
Add documentation about external links

Signed-off-by: Damien Cassou <damien.cassou@gmail.com>
Add documentation about commented lines

Signed-off-by: Damien Cassou <damien.cassou@gmail.com>
Handle comments

Signed-off-by: Damien Cassou <damien.cassou@gmail.com>