[WIP] Refactor: easybib_crontab

 * moved cron parsing logic into a library
 * exit early from provider code to avoid large if/else

 * todo:
   * [x] test (this is just a WIP)
   * [x] discuss this approach
   * [ ] consider introducing `EasyBib::Cron::Entry`
   * [x] add a test to confirm parsing works as expected