v4.0.0

This change improves the experience on touch devices #80 (@fyvfyv), but makes some changes to CSS classes that might break consumer code with customised styling.

• BREAKING: Moved opening/closing flag logic from CSS to JS with touchstart, mouseover and mousemove events
• BREAKING: Get rid of using .show-flag class and :hover selector for .ql-cursor-flag
• BREAKING:.ql-cursor-caret-container has z-index: -1 on touch devices
• BREAKING: The active area on touch devices depends on .ql-cursor-caret-container paddings
• Added extra .hover and .no-pointer classes to .ql-cursor-caret-container to help with toggling visibility state