Add a When() synonym for Context()

[decibel]

I find myself writing a lot of Context()s with a description in the form "when ", ... It(). Enough so that I'm wondering if it'd be worth adding a When() to the DSL, something along the lines of:

    return Context("when " + description, ...)
}```

That way if you write:

```When("something happens", func() {
...
})```

you'd get a "when" in the text output as well.

[onsi]

Makes sense and should be relatively easy. Will try to get to this soon.

[williammartin]

This was done in #386. Hope it works for you.

[decibel]

Works great, thanks for your efforts!