Support adding hooks at the job level, rather than individual steps

e.g.:

``` yaml
name: my-job
plan:
- get: something
- task: run-something
on_failure: {...}
on_success: {...}
ensure: {...}
```

would be equivalent to:

``` yaml
name: my-job
plan:
- do:
  - get: something
  - task: run-something
  on_failure: {...}
  on_success: {...}
  ensure: {...}
```
