Add custom usage message

[KevinHock]

So my bad, I missed this bug when I made the change to run it with -m, but the default argparse usage message no longer fits with how the program should be run. (It says to run '_ main _.py')

python3.5 -m pyt example/vulnerable_code/sql/sqli.py

usage: __main__.py [-h] (-f FILEPATH | -gr GIT_REPOS) [-pr PROJECT_ROOT] [-d]
                   [-o OUTPUT_FILENAME] [-csv CSV_PATH] [-p | -vp]
                   [-t TRIGGER_WORD_FILE] [-l LOG_LEVEL] [-a ADAPTOR] [-db]
                   [-dl DRAW_LATTICE [DRAW_LATTICE ...]] [-li | -re | -rt]
                   [-intra] [-ppm]
                   {save,github_search} ...
__main__.py: error: invalid choice: 'example/vulnerable_code/sql/sqli.py' (choose from 'save', 'github_search')

Hopefully it's something simple like http://stackoverflow.com/questions/21185526/custom-usage-function-in-argparse

[KevinHock]

Wth the above PR it now does

python -m pyt example/vulnerable_code/sql/sqli.py

usage: python -m pyt [-h] (-f FILEPATH | -gr GIT_REPOS) [-pr PROJECT_ROOT]
                     [-d] [-o OUTPUT_FILENAME] [-csv CSV_PATH] [-p | -vp]
                     [-t TRIGGER_WORD_FILE] [-l LOG_LEVEL] [-a ADAPTOR] [-db]
                     [-dl DRAW_LATTICE [DRAW_LATTICE ...]] [-li | -re | -rt]
                     [-intra] [-ppm]
                     {save,github_search} ...
python -m pyt: error: invalid choice: 'example/vulnerable_code/sql/sqli.py' (choose from 'save', 'github_search')