Allow for multiple vrefs

[hakan42]

When https://github.com/KSP-CKAN/CKAN/issues/752 is implemented, it would make sense to allow for multiple vref enhancement steps to occur, e.g. first add metadata from the kerbalstuff vref, then add from the AVC vref.

Maybe even add from multiple AVC files.

This can be done in the same way the Author field is managed, e.g. by turning it from "string" to "onOf".

Some ordering would have to be defined, though, to determine strictly what happens when multiple vref handler suggest data for the same key.

[hakan42]

author configuration from the schema:

        "author" : {
            "description" : "Author, or list of authors.",
            "oneOf"       : [
                { "type" : "string" },
                {
                    "type"        : "array",
                    "items"       : { "type" : "string" },
                    "uniqueItems" : true
                }
            ]
        },