add native context support to Run methods

Currently you can manually bind a `context.Context` to a `kong.Context`, see #48, but this is not as turnkey as I would have hoped. Would it be possible to add an automatic bind, assuming one is not passed to override, for `context.Context` that is cancelled when `kong.Context.Run` returns?

> In the override case, I can see arguments for and against, but it may be useful to subcontext via `context.WithCancel` and explicitly cancel. Need to think through the implications on this one more.

Happy to contribute such a fix, but I thought I would ask if this was wanted first.