Created provider for cron expressions.

[swekaj]

Created provider to produce valid cron expressions, both basic and extended (as indicated by Wikipedia's format section).

Also provided methods for the individual Cron fields.

[fzaninotto]

Thanks for your contribution. In my opinion, it's a bit too specific for Faker. I suggest you host a standalone provider in your own GitHub account (like I did for CompanyNameGenerator), I'll be happy to link to it in the README.

[swekaj]

I'm sorry to hear that, but I went ahead and followed your advice and created swekaj/CronExpressionGenerator.

[fzaninotto]

Thanks, I mentioned it in the README in aca1dac

[swekaj]

Thanks!