Add `validate_between` for numbers and Time

[jwoertink]

This would allow you to validate that either a number or a Time based column sits between some lower bounds and some upper bounds.

Something like this

validate_between age, min: 30, max: 60
validate_between coupon, min: 2.days.ago, max: 2.days.from_now

We actually already have validate_numeric for validating that a number is within some range, so we could just make this for Time only, but I figure someone might reach for a number and wonder why it didn't work... We wouldn't get rid of validate_numeric though because with that you can set only a min or only a max and not care about the other bounds. With this you want to ensure that it's directly between the two.