I've been testing some code that generates timestamps and found that I'm often thinking in terms 'time should be between a and b'. So I think this might be helpful.

Example of how I've been writing my tests:

```go
before := time.Now()
data := DoSomething()
after := time.Now()

assert.WithinTimes(t, before, data.timestamp, after)
```

Without this, I have to use `WithinDuration`, which I find _slightly_ cumbersome to setup and read.

```go
before := time.Now()
data := DoSomething()
span := time.Since(before) /2
midpoint := before.Add(span)

assert.WithinDuration(t, midpoint, data.timestamp, span)
```