Combining DOI and URL fields to provide

Issue Summary

Referring to #3 in #162792: DOI Look up

Wow! I just discovered this module, and it's fantastic what's been done with it!

Anyhow, quick suggestion, referring back to item 3 in the #3 post: Would it make sense to have a URL/DOI field instead of 2 separate ones? It seems to me that the only time a URL would be useful would be when there isn't a DOI, since DOI provides the link when prefixed with "http://dx.doi.org/"

Personally, I ask because I've been prepopulating with the DOI, and just now realized that I can't get the titles to link to the articles (using the setting in Preferences > Links) unless I go back and manually copy the DOI URL into the URL field. Perhaps the easier solution would be to read the DOI if the URL field is empty?

Anyhow, thanks for all you do!