Cite by title / custom anchor text

[cboettig]

enable the ability to cite by article title using crossref API, which will return a DOI given citation information (which could potentially other data than title as well). see http://search.labs.crossref.org/

Would probably need better error handling mechanism in case unique citation could not be resolved.

Should probably query against an external CrossRef R package. Potentially coming through ropensci.

[cboettig]

In addition to a default of citing by title if author cannot be found, should provide an additional optional argument to citet citep that would allow user to define the anchor text themselves.