"999 is enough for anybody" triple-zero padding (#18)

When I hit 999 exercises, I will finally have reached the ultimate
state of soteriological release and no more exercises will be needed.
The cycle will be complete. All that will be left is perfect quietude,
freedom, and highest happiness.

exercises/013_while3.zig

@@ -0,0 +1,33 @@
+//
+// The last two exercises were functionally identical. Continue
+// expressions really show their utility when used with 'continue'
+// statements!
+//
+// Example:
+//
+//     while (condition) : (continue expression) {
+//
+//         if (other condition) continue;
+//
+//     }
+//
+// The "continue expression" executes every time the loop restarts
+// whether the "continue" statement happens or not.
+//
+const std = @import("std");
+
+pub fn main() void {
+    var n: u32 = 1;
+
+    // I want to print every number between 1 and 20 that is NOT
+    // divisible by 3 or 5.
+    while (n <= 20) : (n += 1) {
+        // The '%' symbol is the "modulo" operator and it
+        // returns the remainder after division.
+        if (n % 3 == 0) ???;
+        if (n % 5 == 0) ???;
+        std.debug.print("{} ", .{n});
+    }
+
+    std.debug.print("\n", .{});
+}