added pre_process_block_element() method hook

This method allows to pre-process markdown blocks before they are
rendered to html.

This allows for example to process code blocks for syntax highlighting
but is very flexible as it allows to work with any block type in
arbitrary ways.

I tried to adopt to your code style, hope everything looks as it should. Btw, what is the reason you are not using phpdoc standard for class and method description?