Context and options for callbacks

I need to be able to refer to the options of the task in my after-callback to be able to keep my path declarations DRY.

This PR calls both the afterEach-callback and after-callback with the context of the task and also passes the options object to both callbacks.

The addition of the context is more of a "bonus". When writing a callback for grunt-md5 you typically write it in the Gruntfile.js. So naturally, when I wrote my first callback, I expected the context in my callback to be the same as the context for the task. But maybe this is just me. Passing the options object directly, without setting context, would be enough though. But I figured the task context couldn't hurt, unless people start doing `this.options()` inside their callbacks. But I think that is more of a documentation issue than code.

Hope I make sense :)