make ConfigManager from documentation part of api

[gepron1x]

The ConifgManager example simplifies use of DazzleConf a lot. I copy and paste it to projects i work on. So, why not to include the class in the api itself?

[A248]

The only thing we'd need to consider is that the wiki example is opinionated. It uses System.err.println and ex.printStackTrace, which libraries should never do. It also depends on SnakeYaml. The current ConfigurationHelper is supposed to reduce as much boilerplate as possible without requiring opinionated handling.

More importantly, however, for 2.0 we are probably going to couple configuration loading more closely with defaults handling (#36). Then we'd provide a hook for invalid values (#42), which corresponds to the second catch block in the wiki example. This might obsolete the use of classes like ConfigurationHelper entirely, or at the least, reduce their volume of code.

Perhaps we should consider a higher-level module such as dazzleconf-helper in addition to the core and format-specific modules. Then the base abstractions would reside in core, whereas more opinionated presets would come in the helper. Although, I must say, DazzleConf has always been opinionated, to an extent, by design.