Add modal style preview.

[alexdraconian]

Currently refnotes supports "tooltip" and "popup", which requires mouse curser to over refnote indicator. Instead of this, please add new preview style "modal" works like this:

• When user clicks it, modal box appears to show footnote content.
• When user click "X" or "close" button or click outside modal box, modal box will close.

With this way, mobile users don't have to scroll back and forth to see what footnote says.