Add isWeekend code snippets

snippets/isWeekend.md

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+### isWeekend
+
+Results in a boolean representation of a specific date.
+
+Pass the specific date object firstly.
+Use `Date.getDay()` to check weekend then return a boolean.
+
+```js
+const isWeekend = (t = new Date()) => {
+  return t.getDay() === 0 || t.getDay() === 6;
+};
+```
+
+```js
+isWeekend(); // 2018-10-19 (if current date is 2018-10-18)
+```