Reject invalid dates

[erikkemperman]

cf. gulpjs/vinyl-fs#191

[phated]

This would normalize to a timestamp. So the method name would be weird. I think, based on #4, that it would take lenient input (dates, numbers, number objects) and return a Date object. We could mirror that with a .timestamp() method that returns the number timestamp.

Also, do you think we should use the vali-date module to do date object validation?

[erikkemperman]

This PR doesn't change the return type, just the boolean function to decide accept/reject of the given value. In response to the vinyl-fs issue about consistent dates.

Looking at this though, got me thinking about lenient in / strict out, and then I made that issue here, so I guess my timing was confusing but this PR is separate.

I have something working for that other issue as well but would like to add a bit more.

[erikkemperman]

PS vali-date would not be necessary I think. Wasteful even, because it checks validity by actually coercing to Date... Which it then throws away and returns true or false. For us to coerce again.

[phated]

Oh yeah, I was thinking about that method incorrectly. This looks good. I'll merge and bump patch version.

[erikkemperman]

Thanks!