Allow using context.Context instead of a channel

[mpldr]

It would be great to have the possibility to use a context instead of a channel in function calls, since they pretty much do exactly what the stop channel is supposed to do as well. With network libraries it's more common to use a context which also allows for easy implementation of timeouts and cancellation that's passed through multiple child contexts.

• breaking the API can be avoided by adding a *Ctx function which just accepts context.Context instead of a stop channel
• code duplication can be avoided by just calling the *Ctx-function from the channel-function