use docopt to enhance arg parse and use pattern presentation

[hupili]

use docopt. We can simply write typical use patterns in the code. This gives more useful information for first timers.

redirected from #13

[mfrasca]

reasons for doing so: »PEP 257 recommends putting help message in the module docstring.« »[using docopt,] The option parser [can be] generated based on the docstring« as they at docopt state: beat that!