Method 'parse' in package module 'commands' now reads a given input,
recognizes input matching registered commands, extracts argument values
and calls bound handler. Some default commands are registered in the same
module.
The 'arg' module has been renamed to 'arguments', because of names.

TODO: The input command parser might switch to an inappropriate command
	syntax path the moment it reads an argument value that could be fit
	to more than one argument. Parsing should probably be recursive in order
	to be able to choose which command sytnax an input matches in its entirety.
	Maybe have a look at regular language parsing algorithms and/or into
	what we learned at school about parsing/regular languages.