* add VOC visualize

* fixn ssd_mobilenet_v1_voc.yml

* use default label

* clean TestFeed dataset config

* fix voc default label

* fix format

* fix as review

* revert voc default

* use defult label for all

* enable batch size != 1

* Tweak command line scripts

enable fine grained control of command flags parsing
switch to attribute style of accessing config options

* Break down visualization function

decouple it from IO operations

* Move flag parsing out of `main()`

* Fix a bug where `None` is returned instead of `{}`

* Rename `save_xxx` to `output_xxx` in command line flags

could be confusing since checkpoint is stored in `save_dir`

* Support image file extensions in upper case

* refine infer with args

* remove samples

* fix as review

* refine code

* refine args

* move get_test_images to infer.py

* add visualize log

* fix images = []

* fix args

* refine infer.py

* fix yolov3_r34.yml

* Unified object detection framework based on PaddlePaddle.
* Include algo: Faster, Mask, FPN, Cascade, RetinaNet, Yolo v3, SSD.