visualize retrieval output

[snapfinger]

The following changes are made:

1. visualize retrieved images to give direct sense
2. separate feature extractor and indexing process
3. add command line parsing to enable customed image directory and output index file name
4. add corresponding user guide in readme file

[willard-yuan]

@snapfinger Very thanks for the pull request to improve the project. I have tested it and it's OK.

[dgtlmoon]

@snapfinger see https://github.com/dgtlmoon/flask-keras-cnn-image-retrieval for an example of improving the matlib plot output