Support for CITATION files

[mfenner]

As described in https://www.software.ac.uk/blog/2016-10-06-encouraging-citation-software-introducing-citation-files.

The R community is doing this for years. These recent issues suggest improvements:

• Automated JOSS submission ropensci/unconf17#46
• R Package citation tools ropensci/unconf17#24

@cboettig I added your comments here for future reference.

[mfenner]

Also look at https://github.com/duecredit/duecredit/