Expose all formaters as services

Right now, it's not damn easy to configure a custom formatter
for an handler, because the developer should create a new service.
So, only for formatter that does not depend on external
configuration, we can use directly monolog formatter:

    heka:
        type: socket
        connection_string: "%heka_endpoint%"
        timeout: 1
        connection_timeout: 1
        formatter: monolog.formatter.json_formatter