If the description of an option contains the single word sentence
'Required.' it is understood as a required argument and automatic
validation will make sure that it is set or end the script by calling
help.
Alternatively, if the argument within the __usage string is {arg}
instead of [arg] it is understood as a required argument and automatic
validation will make sure that it is set or end the script by calling
help (even if 'Required.' is not in the description).
Closes#22
