Command-line arguments parsing library.
=======================================

This module is inspired by parse-options.c (git) and python's argparse
module.

Arguments parsing is common task in cli program, but traditional `getopt`
libraries are not easy to use. This library provides high-level arguments
parsing solutions.

The program defines what arguments it requires, and `argparse` will figure
out how to parse those out of `argc` and `argv`, it also automatically
generates help and usage messages and issues errors when users give the
program invalid arguments.

Features:
=========
 - handles both optional and positional arguments
 - produces highly informative usage messages
 - issures errors when given invalid arguments

Examples:
=========

    #include "argparse.h"

    static const char *const usage[] = {
        "test_argparse [options] [[--] args]",
        NULL,
    };

    int
    main(int argc, const char **argv)
    {
        int force = 0;
        int num = 0;
        const char *path = NULL;
        struct argparse_option options[] = {
            OPT_HELP(),
            OPT_BOOLEAN('f', "force", &force, "force to do", NULL),
            OPT_STRING('p', "path", &path, "path to read", NULL),
            OPT_INTEGER('n', "num", &num, "selected num", NULL),
            OPT_END(),
        };
        struct argparse argparse;
        argparse_init(&argparse, options, usage, 0);
        argc = argparse_parse(&argparse, argc, argv);
        if (force != 0)
            printf("force: %d\n", force);
        if (path != NULL)
            printf("path: %s\n", path);
        if (num != 0)
            printf("num: %d\n", num);
        if (argc != 0) {
            printf("argc: %d\n", argc);
            int i;
            for (i = 0; i < argc; i++) {
                printf("argv[%d]: %s\n", i, *(argv + i));
            }
        }
        return 0;
    }