Make contributors property Data Package V2 complaint

[dtufood-kihen]

Update new_contributor() to be compliant with Data Package v2 contributor spec

• Renames 'role' to 'roles' which allows for multiple contributor roles
• Adds support for givenName, familyName, and organisation fields
• Aligns implementation and documentation with spec recommendations
• No test added: unclear how testing is intended to work, as the package uses custom helper functions instead of testthat

[djvanderlaan]

Also looks good. Thank! This one is a bit more involved, so I have to look at it a bit more in depth. I'll try to do that this week and get back to you.

Same question as with the other pull request: would it be possible to add some basic tests?