Support multiple notifiers

Code is rearranged so that `notify` acts as the top-level option
to specify notifier options.

This allows multiple notifiers (though, only New Relic is currently
supported).

Options in YAML look like:

```yaml
deploy:
  provider: foo
  notify:
    - new_relic:
        license_key: 0123456789abcdef
        app_name: foobar
```