fix(exit): add optional "exit" final arg to Parse()

Users of docopt.go should be able to handle their own return
codes and os.Exit() processing. Calling the Parse() func with
an extra last arg of "false" means docopt.go will not call
os.Exit().

Additionally, the docopt_test.go suite was changed to call the
exported Parse() func instead of the parse() implementation.

Fixes #7.