Disallow invalid tab widths for -w

Previously RDoc::Options used OptionParser::DecimalInteger instead of
Integer for restricting values for the tab width.  Unfortunately the
validator in RDoc uses #to_i to validate the input instead of
Kernel#Integer.  This means no exception is raised for invalid input
such as "-w=3".

Now Integer is used which has the strict validation but also allows
octal, hexadecimal and binary numbers.

Bug reported by Charles Hixson.