feat: add workdays endpoint

Added support for our new workdays endpoint that allows you to pull the
number of working days that occur between two dates.

In addition to the new endpoint:
* chore: upgraded dependencies.
* ci: dropped Node.js 10 because it past EOL.
* ci: swapped Node.js 15 for 16 now that it's out.
* docs: added example of new endpoint.

README.md

@@ -251,3 +251,15 @@ holidayApi.workday({
   days: 7,
 });
 ```
+
+### Workdays
+
+#### Fetch number of workdays between two dates
+
+```javascript
+holidayApi.workday({
+  country: 'US',
+  start: '2019-07-01',
+  end: '2019-07-10',
+});
+```