Parse 'fortnight' as a duration of two weeks

plugins/dates/tests/to-iso.test.js

@@ -156,6 +156,8 @@ test('date-parse :', function (t) {
     ['in 1 year', '2020-02-02T00:00:00.000Z'],
     ['3 days ago', '2019-01-30T00:00:00.000Z'],
     ['2 weeks ago', '2019-01-19T00:00:00.000Z'],
+    ['in a fortnight', '2019-02-16T00:00:00.000Z'],
+    ['1 fortnight ago', '2019-01-19T00:00:00.000Z'],
     ['1 month ago', '2019-01-02T00:00:00.000Z'],
     ['1 year ago', '2018-02-02T00:00:00.000Z'],
     ['at 4pm', '2019-02-02T16:00:00.000Z'],