add support for greycite

[cboettig]

http://knowledgeblog.org/greycite

[cboettig]

citet and citep can now take a URL and generate a citation.

• The URL really can be anything, from a journal page, dx.doi.org page, arxiv/pubmed url, wikipedia page, blog post, etc.
• The method requires more testing, and writing some examples and testthat tests. Leaving open until then.

[cboettig]

Currently uses the bibtex format from Greycite, writes this to a bib file and reads in with read.bib. Simple and avoids introducing additional breakpoints, but parsing the JSON and generating the bibentry manually might be preferable for handling special cases.