Preparing for cffr 1.0.0

[dieghernan]

• [x] #66
• [x] Review pkgdown page
• [x] Add subclasses for person and reference
• [x] Review and refactor internal code
• [x] lintr and codecov https://app.codecov.io/gh/ropensci/cffr
• [x] Add documentation for cff-class
• [x] Update docs
• [x] Update NEWS

[dieghernan]

Prepare for #69