Support returning values of a specific field only

Initially, I implemented this as described in #230.

But with full parameters, the new syntax was counter-intuitive, e.g. `GET /resource?id=XY&format=full&field=publisher` does not return the full resource, but only the values of the `publisher` field.

Since the new result format is syntactically identical to the existing `short` format, it seems to make sense to use a modifier on that instead, e.g. `GET /resource?id=XY&format=short.publisher`.

It also makes more sense semantically, since the field is not a property of the query, but of the result format. 

Finally, it also points users to the option of using this for custom auto-suggest values (which is what the default `short` format is intended for).

Deployed to staging, some sample requests:

http://staging.api.lobid.org/resource?id=HT002189125&format=short.fulltextOnline
http://staging.api.lobid.org/resource?author=Böll&format=short.publisher
http://staging.api.lobid.org/organisation?name=Universitätsbibliothek&format=short.locality