rcrossref v0.7.0

NEW FEATURES

• All text mining functionality moved into a new package: crminer
  https://github.com/ropensci/crminer . Functions that did
  text mining stuff now defunct, see ?rcrossref-defunct (#122)
• All Crossref API requests now using https instead of http (#133)

MINOR IMPROVEMENTS

• replace xml2::xml_find_one with xml2::xml_find_first (#128)
  thanks @njahn82
• replaced httr with crul for HTTP requests (#132)
• Now using markdown for documentation (#135)
• Added more documentation to cr_journals and cr_works
  about what the returned data fields backfile_dois and current_dois
  really mean (#105) thanks @SteveViss

BUG FIXES

• Fix to cr_prefixes to not fail when no results found (#130)
  thanks @globbestael
• Fixed cr_works to allow queries like facet = license:* to be
  passed to facet parameter (was always allowed by Crossref, but we
  neglected to allow it - previously only allowed a boolean) (#129)
• Fixed cr_funders and cr_journals to give back facet data along
  with other data (#134)
• Fix to cr_* functions to check for a missing content-type
  headers and instead of failing, we continue anyway and try to parse
  data as sometimes Crossref doesn't give back a content type header
  at all (#127)