Test paragraphs, list items and img attributes.

By default '{\\\.\s*?([^}]+?)}'.

Allows to add attributes to element of an attribute which contains
the attribute pattern.

Works better, and the html elements get their attributes.

Extra text representing classes is detected and correctly removed.
Adding attributes isn't working yet.

Added support for postMessage's into any iframes for content that needs to do something when a slide is displayed and hidden.

If config.center is false, this change centers slides with the
'center' class.

README: clone via https

add details to README on creating themes

Closes #662.