Adding tests for WW and updating README

Author: Alex Smith

Readme.md

@@ -109,7 +109,8 @@ dateFormat(now, "N");
 | `tt`             | Lowercase, two-character time marker string: am or pm.                                                                                                        |
 | `T`              | Uppercase, single-character time marker string: A or P.                                                                                                       |
 | `TT`             | Uppercase, two-character time marker string: AM or PM.                                                                                                        |
-| `W`              | ISO 8601 week number of the year, e.g. 42                                                                                                                     |
+| `W`              | ISO 8601 week number of the year, e.g. 4, 42                                                                                                                  |
+| `WW`             | ISO 8601 week number of the year, leading zero for single-digit, e.g. 04, 42                                                                                  |
 | `Z`              | US timezone abbreviation, e.g. EST or MDT. For non-US timezones, the GMT/UTC offset is returned, e.g. GMT-0500                                                |
 | `'...'`, `"..."` | Literal character sequence. Surrounding quotes are removed.                                                                                                   |
 | `UTC:`           | Must be the first four characters of the mask. Converts the date from local time to UTC/GMT/Zulu time before applying the mask. The "UTC:" prefix is removed. |

test/test_mask-ww_.js

@@ -0,0 +1,30 @@
+var assert = require("assert");
+var dateFormat = require("../lib/dateformat");
+
+describe("Mask: 'WW'", function () {
+  it("should format '1876-01-12' as '02'", function (done) {
+    var date = new Date("1876-01-12");
+    var d = dateFormat(date, "WW");
+    assert.strictEqual(d, "02");
+    done();
+  });
+
+  it("should format '2013-12-11' as '50'", function (done) {
+    var date = new Date("2013-12-11");
+    var d = dateFormat(date, "WW");
+    assert.strictEqual(d, "50");
+    done();
+  });
+
+  it("should format '2020-03-04' as '10'", function (done) {
+    var d = dateFormat("2020-03-04", "WW");
+    assert.strictEqual(d, "10");
+    done();
+  });
+
+  it("should format '2020-02-01' as '05'", function (done) {
+    var d = dateFormat("2020-02-01", "WW");
+    assert.strictEqual(d, "05");
+    done();
+  });
+});