PFA the patch for graphical explain feature.

It consists of explain.js, __init__.py and demo_explain.html, snap_svg.js and snap_svg-min.js file.

Currently I have placed hard coded JSON data in demo_explain.html file to display graphical output; which will be replace by actual output user get from server in JSON format.

Use below URL to access graphical output:

To test the patch, user can replace plan data (stored in 'data' variable) in demo_explain.html with other valid plan data in JSON format and run the above URL.

Please review the patch and provide comments for the same.

TODO: Integration of graphical explain into sql editor