Adds NewMockFrom to build mocks at a certain time

Not sure if this is something you'd even be interested in, but I've started using clock and needed to build mocks that had specific time ranges.  

I'm also a bit new to go and thought this would be a good learning experience :grinning: 

Thanks Ben!
If this is a change @benbjohnson considers, I think taking a `time.Time` instead would be more general and useful.