rcrossref v0.6.0

NEW FEATURES

• Added support for field queries (see
  https://github.com/CrossRef/rest-api-doc/blob/master/rest_api.md#field-queries
  for information on them). Means that you can query on specific fields rather
  than using query parameter which queries across all fields (#111)
• Now using rappdirs for local storage and caching for cr_ft_text (#106)

MINOR IMPROVEMENTS

• Added to man files where appropriate new 10K max value for the
  offset parameter (#126)
• Added to pkg level man file new rate limit headers included,
  and how users can get to those, via config=verbose() call (#124)
• Better failure modes on input parameters, still more work to do
  surely (#101)
• sleeping now between tests to avoid making crossref rate
  limit gate keepers mad (#125)
• cr_search and cr_search_free are now defunct. They were marked
  deprecated in previous version, and warned of defunctifying, and now
  they are defunct. Similar functionality can be done with e.g., cr_works()
  (#102)
• crosscite is now defunct. The functionality of this function can be
  achieved with cr_cn() (#82)
• cr_fundref is now defunct. Crossref changed their name fundref
  to funders, so we've changed our function, see cr_funders() (#83)
• parameter sample maximum value is now 100, was previously 1000.
  documentation updated. (#118)
• New filters has-clinical-trial-number and has-abstract added to
  the package, see ?filters for help (#120)