refactor: handle the argument parsing in a clearer way

[logic-finder]

proofread/argparse.c

Lines 87 to 110 in 19a9d73

	/*
	FIXME: the following codes have this shape:
	if (arg equals to opt_1) {
	// tests to see if it is fine to take this arg
	// sets the option
	}
	else if (arg equals to opt_2) { ... }
	It seems it is possible to refactor like so:
	opts = [opt_1, opt_2, ...]
	handlers = [opt_1_handler, opt_2_handler, ...]
	for (until the end of args) {
	// checks whether arg is in opts; if exists, gets the index
	// invokes the corresponding handler: handlers[index]()
	}
	void opt_n_handler(optflg_t *of) {
	// tests to see if it is fine to take this arg
	// sets the option
	}
	*/