Perform bibliographic search when unstructured citations do not come with (valid) DOIs. https://github.com/OpenBookPublishers/cit-ex/blob/dad3a67109d8a18d1afebc3fc24eeec16e9302a7/cit-ex/main.py#L68 Something similar to [this](https://www.crossref.org/labs/resolving-citations-we-dont-need-no-stinkin-parser/) but using the crossref library.
