Add datetime validation

[psampaz]

• Tests exist or have been written that cover this particular change.
• doc.go was updated
• en translation and translation tests were updated

Change Details:

This PR adds a new validation that checks that a string value is a valid datetime based on the supplied datetime format. Supplied format must match the official Go time format layout as documented in https://golang.org/pkg/time/

@go-playground/admins

[psampaz]

I will rebase this one

[deankarn]

@psampaz cool was just about to resolve conflicts, will wait for your rebase if you can be done soon.