JSHint: disable unused variable check
JSHint: disable unused variable check

This is nice but until jshint/jshint#1146 is fixed there's no way to
selectively disable W098 for unused function arguments which we wish to
include for documentary purposes.

With this change there are only three remaining warnings, all of which
are actionable.

lidasong2014/openseadragon,goofwear/openseadragon,daverome/openseadragon,avandecreme/openseadragon,Iced-Tea/openseadragon,LarissaSmith/openseadragon,goofwear/openseadragon,openseadragon/openseadragon,Iced-Tea/openseadragon,TMUND/openseadragon,ricochet2200/openseadragon,msalsbery/openseadragon,lidasong2014/openseadragon,mcanthony/openseadragon,avandecreme/openseadragon,daverome/openseadragon,sul-dlss/openseadragon,seanich/openseadragon,shaunstanislaus/openseadragon,msalsbery/openseadragon,ricochet2200/openseadragon,jmnetus/openseadragon,seanich/openseadragon,LafayetteCollegeLibraries/openseadragon,shaunstanislaus/openseadragon,janelia-flyem/openseadragon,rdlester/openseadragon,mcanthony/openseadragon,openseadragon/openseadragon,jmnetus/openseadragon,rdlester/openseadragon,LarissaSmith/openseadragon,janelia-flyem/openseadragon,TMUND/openseadragon