oe-depends-dot: Handle new format for task-depends.dot

The .dot file created by `bitbake -g` changed formats a while ago, which
broke oe-depends-dot.

Also add some useful examples to the --help output.

Signed-off-by: Rusty Howell <rustyhowell@gmail.com>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Steve Sakoman <steve@sakoman.com>