Summary:
- Added command line argument to infer_simple.py to choose a custom output image format.
- Added command line argument to infer_simple.py to choose to output an image even when no object is found in it.
Closes https://github.com/facebookresearch/Detectron/pull/400

Reviewed By: ir413

Differential Revision: D8705039

Pulled By: rbgirshick

fbshipit-source-id: fbcd707821a58004971a88946a17f005545ecab4