creddit: Implement argument parsing system.

This patch adds an argument parsing system in the files opt.c and opt.h.
It can parse for '-' single character arguments, '--' long character
arguments, as well as read a string or int off of the command-line for
one of these arguments if wanted.
creddit: Implement opt.h use in main.c

This patch simply replaces the old system of reading argc and argv
directly with using opt.h to handle parsing the arguments, and then
simply handles the output from it.