Add API for `markAsSaved`

[Fleker]

Allow programmatically a user to save articles for later.

[Fleker]

I extended the object with this:

// Add custom read later method
f.markSaved = (ids, cb) => {
  if (typeof ids === 'string') {
    ids = [ids]
  }
  return f._request(cb, '/v3/markers', 'POST', {
    entryIds: ids,
    type: 'entries',
    action: 'markAsSaved'
  })
};