Added liquid templating and migrated first agents

Like discussed in #255 I added liquid templating. The two main benefits are that it is now possible to emit customized events which HTML formatting and that the EventFormatterAgent is not needed anymore for simple use cases (for example send Basecamp updates to a Hipchat channel).

At the moment just Hipchat and EventFormatter Agents are migrated because I am using them actively and thus was able to verify the migration worked.
I am not sure if I covered every use case of JSONPaths in the `LiquidMigrator` so it would be great if you could check if it would work with your agent configurations.