WebSubmit: doilookup function in webinterface

* Adds doilookup function to websubmit_webinterface that uses the
  crossrefutils and returns a dictionary with MARC codes as keys and
  values from crossref website as values (that can be used in
  submissions to prefill some fields based on the DOI).

Signed-off-by: Sebastian Witowski <sebastian.witowski@cern.ch>
`invenio.containerutils` tries to import `defaultdict` from `collections`, if available, so the above try/except clause is not necessary.