PHP5.3: Replace HTML with placeholders as it's inserted

[Kroc]

As part of moving to PHP5.3, large portions of ReMarkable can be re-architectured. ReMarkable limited itself somewhat due to the restriction of one overall function. With anonymous functions, we can rethink some of the complex regex sections and deal much better with the multi-layered aspect of markup.

At the moment, markup collision with HTML is very carefully handled and avoided. Whilst PRE / CODE are masked out right at the beginning, as more HTML gets added by headings and other markup, chances of collision may increase. Using PHP5.3 anonymous / nested functions, it will be possible to replace HTML tags with placeholders after each instance where HTML tags get inserted. This should reduce collision and reduce the need for regex edge cases.