Allow to specify extras keys via a custom .json file

[paulmueller]

Different installations of DCOR (e.g. hospital vs. research) need different metadata sections and keys. Instead of programatically writing a plugin, it should be possible to activate custom metadata trees.

• [x] introduce json writer/parser for metadata sections and keys
• [x] incorporate custom metadata via external json files in plugin.py
• [x] make the web interface expandable for those custom metadata
• [x] ~add a ckan command to manage those metadata sections~ add ckan configuration option for setting json directory

[paulmueller]

36bb233152e5a3285dd900f9fb5922d9e2d84865