Add note about using reveal.js from a CDN in the README.
Add menu entry to export and browse HTML file.

Add function org-reveal-export-to-html-and-browse that call
org-reveal-export-to-html and browse generated file.

Add menu entry `C-c R B` to export and browse HTML file.
Merge pull request #29 from Sliim/menu-entry-browse-file

Add menu entry to export and browse HTML file.
Merge pull request #28 from punchagan/reveal-CDN-readme

Add note about using reveal.js from a CDN in the README.