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>
diff --git a/client.c b/client.c
index 50c1704..93c7103 100644
--- a/client.c
+++ b/client.c
@@ -733,7 +733,7 @@
 		log_info("\nDisk stats (read/write):\n");
 	}
 
-	print_disk_util(&du->dus, &du->agg, terse_output);
+	print_disk_util(&du->dus, &du->agg, output_format == FIO_OUTPUT_TERSE);
 }
 
 static void convert_jobs_eta(struct jobs_eta *je)