cffr 0.3.0

• preferred-citation is only produced when a CITATION (R) file has been
  provided with the package (#37).
• Improve email handling on authors.
• Add cff_read() function. This functionality was already implemented on
  cff() but new function added in order to provide clarity.

What's Changed

• Improve preferred-citation handling by @dieghernan in #38

Full Changelog: v0.2.3...v0.3.0