Add isWeekday code snippets

snippets/isWeekday.md

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+### isWeekday
+
+Results in a boolean representation of a specific date.
+
+Pass the specific date object firstly.
+Use `Date.getDay()` to check weekday then return a boolean.
+
+```js
+const isWeekday = (t = new Date()) => {
+  return t.getDay() >= 1 && t.getDay() <= 5;
+};
+```
+
+```js
+isWeekday(); // true (if current date is 2019-07-19)
+```