Updates to match_citations()

[allenbaron]

• Skip matching when one vector is all NA and include message indicating skip.
• Add (optional?) output of number of matches identified by each publication type (when more than one).
• Update message to make sort order more explicit (e.g. pmid > pmcid).
• Ensure matches by DOI are case insensitive.

Also consider:

• Add warning when an ID column is found in both data.frames but no matches are identified.