Add rrule from a git commit hash

[tgrosinger]

A required fix is added in jkbrzt/rrule#444
Until that PR is merged, we will use the source commit directly.

[tgrosinger]

This is not yet functional. The tests fail:

$ ./yarn test 
yarn run v1.22.5
$ jest
 FAIL  tests/task-line.test.ts
  ● Test suite failed to run

    src/task-line.ts:15:34 - error TS2307: Cannot find module 'rrule' or its corresponding type declarations.

    15 import RRule, { Frequency } from 'rrule';