Update cli.py

[MichaelQuaMan]

Set default to a string, instead of "False", which causes the type to be "BOOLEAN".

#21

Error: Invalid value for '--source': 'path_to_project/' is not a valid boolean.

Options:
  --source BOOLEAN  Try to find cycles in the path provided.
  --ignore TEXT     Comma separated directories that will be ignored during
                    analysis.