Refactor the architecture to support pre/post-processing plugins

[0nko]

The core Aztec code currently contains features which are WordPress-specific and there are features missing don't belong directly in the library itself. A new plugin architecture is required that will allow the implementation of the following features outside of Aztec:

• Removal of special comments into a separate plugin - PR
• Implementation of Shortcodes, which are also WP-specific - PR
• Removal of Calypso paragraphs into a separate plugin