Make hooks a part of the dependency graph.

At the moment, the lifecycle around hooks are pretty basic:

1. `pre_build` runs before stacker does anything to stacks
2. `post_build` runs after stack does everything to stacks

It would be nice if hooks were just a part of the dependency graph, like stacks are, so that you can reference outputs from hooks, and use them as inputs to stacks and vice/versa.

For example:

```yaml
hooks:
- name: "ecsCluster"
class_path: stacker.hooks.ecs.create_clusters
args:
clusters:
- "staging"

stacks:
- name: "app"
class_path: blueprints.App
variables:
ECSCluster: ${hook_data ecsCluster::ClusterName}
```

Instead of stacker executing the hooks in a separate step, stacker would just execute the hooks as it walks the dependency graph (`app` depends on `ecsCluster`).

I'm not even sure if "hooks" would be the right name for this if it was supported, since it'd be more like a pseudo/virtual stack.