Add utility to reformat automatically

macro to 80 characters.

example/ex-string01.c

@@ -0,0 +1,43 @@
+#include <stdio.h>
+#include <string.h>
+
+#include "m-string.h"
+
+#define MAX_COLUMN 80
+
+static void format(FILE *f)
+{
+    string_t s;
+    string_init(s);
+    while (string_fgets(s, f, STRING_READ_PURE_LINE) == true)
+    {
+        size_t n = string_size(s);
+        if (n > 0 && string_get_char(s, n-1) == '\\' && n < MAX_COLUMN) {
+            string_left(s, n-1);
+            while (n < MAX_COLUMN-1) {
+                string_push_back(s, ' ');
+                n++;
+            }
+            string_push_back(s, '\\');
+        }
+        printf("%s\n", string_get_cstr(s));
+    }
+    string_clear(s);
+}
+
+int main(int argc, const char *argv[])
+{
+    if (argc < 2)
+    {
+        fprintf(stderr, "ERROR: Missing argument filename.\nUSAGE: %s filename.h\n", argv[0]);
+        exit(1);
+    }
+    FILE *f = fopen(argv[1], "rt");
+    if (!f) {
+        fprintf(stderr, "ERROR: Cannot open filename '%s' .h\n", argv[1]);
+        exit(2);
+    }
+    format(f);
+    fclose(f);
+    exit(0);
+}