Use docopt for parsing commandline args

The program now uses docopt from docopt.rs to parse the commandline
arguments and options.
Use costum docopt Config so we can display version

In order to use docopts inbuilt facilities for displaying the
version number, we need to provide our own costum Config struct.
It is pretty much the same as the default, except we write provide
the version.
Add two more commandline options

Also bump the version to 0.3.0