Fix argparser, add MBIPED submodule, and improve README

[maxbrom]

1. Fix the parse_args() function to actually allow the user to input arguments through the command line (outside of the --choose_test_data flag).
2. Add the MBIPED project as a submodule to DexiNed. Since the MBIPED project is required in order to use the BIPED dataset for training, it should not require a separate repository in order to use it.
3. Improve README readability and instructions. Add additional instructions related to the argparser and MBIPED submodule.