Show help with `-h`, better usage

[stephencelis]

$ swiftcov -h
Unrecognized command: '-h'. See `swiftcov help`.
$ swiftcov help
Available commands:

   generate   Generate test code coverage files for your Swift tests
   help       Display general or command-specific help
   version    Display the current version of SwiftCov

Usability, please! :wink:

And perhaps usage for the help command?

---

$ swiftcov help generate
Generate test code coverage files for your Swift tests

[--output (string)]
    Folder to output the coverage files to

[--threshold 1]
    Threshold value of max hit count (for performance)

[--debug]
    Output very verbose progress messages

The above needs to document the trailing --, perhaps with a general usage.

[kishikawakatsumi]

It seems great. I'll try to change it. Thank you for suggestion!