support for URL citations, linked cites, semantics, tooltips, and more

v0.4-0
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* Support for citations by URL.  Paste the full URL of any academic
  journal or other webpage.  Uses the Greycite API
* Introduces linked inline citations as an option (previously active
  only for CITO links).  Inline citation text will be linked directly
  to the article by DOI or URL.  Toggle on or off using cite_options
  globally, or passing `linked=FALSE` to the calls to `citet` or `citep`
* Introduces tooltips option. Mouse over an inline citation and see
  the full citation information.  * Introduces `cite_options`, to toggle
  certain settings such as tooltips, linked inline text,

* in-line link will use URL if a DOI is not found
* `cite` is now exported to the namespace.  Useful to return a bibentry object given a DOI or URL.