Preview plugin endpoint

[dominik]

Users could write ckan extensions that offer previews for a certain datatype.

Advantages:

• We can say that you can add your own previews
• Every preview would have to define it's own dependencies
• External development (community creates new extensions?)

Questions to answer:

• Specification of the interface
• Precedence of previews for the same data types
• Plugins only define the datatype that they are responsible for or do we call a function that returns whether the extension is responsible.

As far as I can see, this should be fairly easy to implement by following these instructions: ​http://docs.ckan.org/en/latest/writing-extensions.html#writing-a-plugin-interface