Add fake.futureDate() and fake.pastDate()

[serhalp]

Occasionally we have a need to generate a date where the only requirement is that it be in the past or in the future. This requires a bit of inexpressive boilerplate each time.

This proposes to add:

• fake.futureDate(): generates a JavaScript Date later than Date.now()
• fake.pastDate(): generates a JavaScript Date earlier than Date.now()

Alternatively, we could nest these under fake.date.{future,past}? But that would require either making a breaking API change to fake.date() or doing that weird pattern where fake.date is simultaneously a function and an object, which I'm not a fan of.

(Ideally, it would be cool to validate somehow that when these two methods are used, the clock is stubbed, since such a thing should never be used with nondeterministic, actual clock time, but that sounds like something that would be a nightmare to implement since we'd have to make some hacky assumptions about stubbing libraries, etc. It probably isn't worth it.)