Refactor examples/sample.py

[suzil]

• from builtins import input allows for Python 2/3 compatibility while keeping Python 3 syntax
• click is an alternative to the built-in argparse which is just a cleaner, easier-to-read syntactic sugar imo
• I renamed src to source and tgt to target as I think this makes it much easier to read
• I turned the script into several functions so that it is easier to follow what is going on
• I moved example usage documentation to a docstring (more convenient to have it there)
• checkpoint_path was defined twice so I made it so that it is only defined once
• Training now allows for the user to exit ^C and move directly to the interactive model prediction loop