[Highlight] Add interfaces for highlight event

[mahavir155]

Add interface for highlight event so developer can sync with his own server if required

To implement above, use the FolioReader class created in #27.

1. Create HighlightInterface class which implements getter of all required variables
2. registerHighlightListener(HighlightListener highlightListener)

HighlightListener
onAddHighlight(HighlightInterface highlight)
onRemoveHighlight(HighlightInterface highlight)