Support for cron expressions

Hi,

Nice library. I used it for a while now.
But now I need more complex cronjobs. For example monday and tuseday at 20:00.
People have developed those cron expressions:
https://crontab.guru/#5_4_*_*_2,5
They enyble you to define even complex cron schedules compact and fast.
Is there already support for this? Or would you like to add it?
schedule.every().parse_expression("5 4 * * 2,5").do(job)
Something like that would be verry handy.

Start by reviewing the existing schedule.every() API and the proposed parse_expression("5 4 * * 2,5") entry point, then compare the requested behavior with the linked crontab.guru expression. Done means agreeing on and implementing support for complex cron schedules such as Monday and Tuesday at 20:00, with verification of the resulting job timing.

Written by the indexing model from the issue text.