Extend help command to list modules with brief explanation

[nichtich]

For instance

catmandu help export           # describes Catmandu::Cmd::export
catmandu help export [to] JSON # describes Catmandu::Exporter::JSON
catmandu help exporters        # should list all exporters

catmandu info --exporter already gives exporters but duplicated if multiple of same name are found in INC and for help another format would be better, e.g.

 JSON: a JSON exporter
Multi: export you data to multiple exporters
 Stat: a statistical export
  TSV: a tab-delimited TSV exporter

By the way Catmandu::Cmd::info needs some extension anyway.

[nichtich]

catmandu help export and catmandu help export <exporter> are already implemented, only catmandu help [exporters|importers|fixes|stores|validators] based on catmandu info is missing. The latter is not suitable for easy help but for further processing of the result list.

[nichtich]

• catmandu help fixes (could be extended with some additional help text)
• catmandu help exporters
• catmandu help importers
• catmandu help stores
• catmandu help validators

The implementation could also be used for #261