Uw flexible reporter config

Cleaned up the reporter setup.

* The old `{report, [{reporters, ...}, {subscribers, ...}]}` options are supported, but also the simpler forms `{reporters, ...}` and `{subscribers, ...}`

* Other applications can configure reporters and subscriptions using `{exometer_reporters, ...}` and `{exometer_subscribers, ...}` environment settings.

* Reporters can now be started as `{ReporterName, Options}`, where supported options are `{module, Mod}` and `{registered_name, none | atom()}`. For BW compatibility, the `module` option defaults to `ReporterName`, and if `registered_name` is not given, the reporter automatically registers as `ReporterName`.

* Improvements of the lager reporter output