EventFormattingAgent: Provide access to upstream agent

Excerpt from the commit log of e55bb4a:
> With this enhancement you can, for example, embed the site URL in an
> event created by WebsiteAgent by interpolating like
> "{{agent.options.url}}", or add a message prefix depending on the type
> of upstream agent using the "case" construct of Liquid.

I believe user cannot do harm via AgentDrop, but let me know if I'm missing something.