Allow {style} macros to be nested

Currently the WYSIWYG editor will fail if a user styles a block and then adds a style on some elements inside that block. For example the following will not work correctly:

{style:type=div|align=center}
Hello {style:type=span|font-size=24px}sir{style}

, this is a test

{style}

To solve it, make the style macro support recursion.