Default to current directory if --path is not specified

[polyfractal]

--path is a required flag at the moment, but Athletic should default to the current working directory if none is provided by --path.

[bishopb]

I consider requiring paths A Good Thing. I'd rather see the CLI signature be, though:

php vendor/bin/athletic [options] <path> [path...]

Thus:

php vendor/bin/athletic -b vendor/autoload.php ./tests ./vendor/bishopb/stencil/tests