WIP ext: CrossRef Flask extension

* Queries DOI record data from the database or CrossRef as JSON.

* Closes #1906.
If you change CrossRef client from OpenURL API to REST API, then it might be interesting to do this in the "master" branch too, in order to have similar PROD vs LABS record massaging treatment.  

CC-ing @switowski who implemented `crossref2marcxml.xsl` originally.