Expand logging options

* Add support for the log config format introduced in [DropWizard 0.7.0](https://dropwizard.github.io/dropwizard/manual/core.html#id4), which will allow configuration of multiple loggers of the same type.
* Deprecate the existing log config format (backwards compatibility preserved).
* Add support for filtered appenders, which wrap another appender and limit the log messages it receives by logger name.