Add offset option

[Sleavely]

As a consumer of next-monthly I don't want to have to figure out how to add or subtract months to show upcoming or past billing dates.

Suggested additional option and its behavior:

const nextBillingDate = nextMonthly({
  from: '1990-01-07T01:29:03.999Z',
  now:  '2022-08-23T12:29:08.551Z',
  offset: -1, // <- This is new. Could default to 0
})
// 2022-08-07T01:29:03.999Z instead of 2022-09-07T01:29:03.999Z