adding the abilty to load plugins installed as npm packages

example webhooks plugin at: https://github.com/mintbridge/uptime-webhooks

- the plugin should be installed with npm, then enabled in the
config, e.g.
plugins:
  webhooks:
    package: uptime-webhooks
    event:
      up:
        - 'http://localhost:8082'
        - 'http://www.example.com/do/something'
      down:
        - 'http://www.example.com/warn/somebody'
      paused:
      restarted:
    dashboardUrl: 'http://localhost:8082'