Crossref API is metadata-only, so print warning to users that query with --pdf/xml/supp

[rossmounce]

I know it is mentioned at the top of the repo that the Crossref API is metadata only but even so, it would be nice to print a warning message to users that persist in trying to query that API for pdf/xml/supp

e.g. "--pdf was ignored, the Crossref API returns metadata only"

As it currently is "info: Downloading fulltext PDF files" is a little misleading...

getpapers --api crossref -o cr-res --filter "type:journal-article,prefix:10.7554,from-pub-date:2016-01-01,until-pub-date:2016-02-01" --pdf
info: Searching using crossref API
info: Found 84 results
info: Saving result metadata
info: Full CrossRef result metadata written to crossref_results.json
info: Individual CrossRef result metadata records written
info: Downloading fulltext PDF files