move all highlight-handling to plugin

Readme, index - updated with new config syntax
LRTEditor.HighlightPlugin
        - now contains everything related to highlighting, instead of
          calling editor.highlight()
LRTEditor.UndoPlugin
        - replaced highlight call with reformat()
        - bugfix where onKeydown might reset $ignore
LRTEditor
        - callback is now a config-option, added config-merger