feat(core): implements the every-at cron syntax

[barino86]

This would close out the #44 issue that I created the other day.

[abichinger]

Hello @barino86,

thank you for the PR. It seems like crontab does not support this syntax so I would prefer if this feature was disabled by default.

Which schedular are you using?

[barino86]

We are using a kind of home-grown scheduler. We use this library in the UI of our application to help end users build the cron expressions they need, then we use a python library croniter to determine the next scheduled date time based on the cron expression.

[barino86]

How would you like me to turn it off by default, possibly have a prop that can enable it?

[abichinger]

Yeah, a boolean prop would be good.

[barino86]

Let me know if there is a different way you would like to see this implemented.

[barino86]

Oops, going to fix the test.

[barino86]

Fixed.

[barino86]

I want to make that prop more dynamic by adding a watch and setting it so that it can be changed on the fly if it needs to be. working on the change for that.

[barino86]

Prop is now dynamic and has been added to light and vuetify as well. I think this is ready once you call it good.

[abichinger]

Looks good. Thank you for your contribution.

[barino86]

Thank you for being open to a new feature on an older version.