Support callbacks to be executed within contexts.

[lwoodson]

There should be some defined callback interfaces that can be used as anonymous instances passed to Gumshoe for execution within a context. The interface would be something like this:

Java 7:

Gumshoe.get().context("my context").put("a", 1).thenDo(new InContextCallback() {
    public void exec() {
        // do a bunch of stuff
        // emit some events
        // etc....
    }
});

Java 8:

Gumshoe.get().context("my context").put("a", 1).thenDo(() -> {
    // do a bunch of stuff
    // emit some events
    // etc....
});

The thenDo method would start the context and make sure it is cleaned up correctly, either with a finish or fail if an exception is encountered when executing the callback.

[lwoodson]

I now prefer having an overloaded start method for this rather than thenDo. Reads better and has a smaller API footprint.

context.start(() -> {
  // do something
});

Gumshoe.get().context("my context").put("a", 1).start(() -> {
  // do something
});