hook up insertTags(), removeTags() and getTags() in ASSLineContents; add reverse processing to ASSLineContents callback to allow for negative indexes to be used in insertTags()
ASSTags: add reverse mode to ASSLineTagSection:callback()
ASSTags: support negative indexes in ASSLineTagSection:insertTags()
ASSTags: don't error out when ASSLineContents:insertTags() is used without an index argument