Suggested Validator: DateTimeRange

Hello Alec,

first of all: thank you for your wonderfull library! :+1:

As the maintainer of [projecthamster](https://github.com/projecthamster), handling and validating all kinds of time/date/dateime related data is somewhat of a big thing for me. One rather common task is to handle not only individual datetimes but ranges. That is, to make sure that some data
- is actually a range
- and that the range passes basic sanity checks: start < end

Is this something you would be interested in as a PR (same with regards to a ``validators.Time`` funciton similar to ``validators.Date``?