[#1002] Allow can_preview to return a dict
[#1002] Better preview plugin selection
[#1002] Fix failing tests
[#1002] Code cleanups to catch fixable plugins and reduce code duplication
[#1002] Code fixes to changes
[#1002] Update text/pdf previews to new method
[#1002] Some cleanups of the plugins since I was there
Merge branch '1002-resource-preview-improvements'