CellExternalCopyManager plugin

Added from https://github.com/Celebio/SlickGrid. Improved
getDataItemValueForColumn performance by using formatter. Added
includeHeaderWhenCopying to include header when copying. Fixed Copy code
to work with row selection model.
Merge remote-tracking branch 'ronmichael/master'

Conflicts:
	plugins/slick.cellexternalcopymanager.js