Add --output-format command line option

Add --output-format command line option

Right now we have normal, terse, and json output. Lets add a parameter
that controls how to output results, instead of having options for
both terse and json outputs. If we are going to add a 4th output
type in the future, it's only going to get more messy.

Signed-off-by: Jens Axboe <axboe@kernel.dk>