Use plural abbreviation 'eds.' in CMoS author-date

It is specified in section 15.9 that the abbreviation in author-date when there are multiple editors is 'eds.' Please let me know if there is some other function for this, but I haven't been able to find it.
Merge pull request #915 from adunning/patch-1

Use plural abbreviation 'eds.' in CMoS author-date