Support merging known-good metadata from OJS into stack output

[axfelix]

In most cases, authors and/or editors will manually input article metadata into OJS at submission time and it will be verified be at least one other person, thus making it generally more reliable than metadata extracted from our stack. Thus, the plugin should support merging this metadata from OJS back into output from our stack.

It might be easiest to do this by altering the POST API call to our stack to allow additional metadata to be submitted which is then added during the Merge phase, rather than doing the merge as part of the plugin itself.

[axfelix]

Implemented on XMLPS side in pkp/ots#82