Add support for built-in search plugin

[ggrossetie]

reveal.js 4.x (and 3.9.2) includes a built-in search plugin. By pressing <ctrl> + <shift> + <f> you can search slide content.

I think we should add an attribute to enable/disable this plugin:

:revealjs_plugin_search: enable

Since this plugin was not supported until now I think it should be disabled by default (i.e., this plugin should be explicitly enabled).

[edl7878]

Have been realized in Issue 370 support reveal js 4 x.