Add check/3, parse_and_check/3, and complete test cases

The functions now allow to specify options:
    [help | {skip, [Name::atom()]}]

	help - instructs getopt:check/3 to return 'help'
		when "-h" command line option is given and
		option specification contains definition of 'help'.
		This is needed to short-circuit the checker of
                required options when help is requested.

	{skip, Names} - tells the checker to skip checking given options

Added test cases for format_error/2 and the new functions mentioned above