rcrossref v0.5.8

NEW FEATURES

• Addded an RStudio Addin for searching for citations. See ?rcrossref for
  more. Addin authored by Hao Zhu @haozhu233 (#114)
• New function cr_abstract() that tries to get an abstract via XML provided by
  Crossref - NOTE: an abstract is rarely available though (#116)

BUG FIXES

• Fixed bug in cr_cn() where DOIs with no minting agency found were
  failing because we were previously stopping when no agency found.
  Now, we just assume Crossref and move on from there. (#117)
  thanks @dfalster !
• Fix to cr_r() when number requested > 100. Actual fix is in
  cr_works(). Max for sample used to be 1000, asked this on the
  Crossref API forum,
  see CrossRef/rest-api-doc#146 (#115)
• Fix to cr_journals() in internal parsing, was failing in cases
  where ISSN array was of length zero