Add support for the APT (Almost Plain Text) syntax

APT: http://maven.apache.org/doxia/references/apt-format.html

One interest for this syntax is to be able to export wiki pages to Maven and then use the Maven Site plugin to generate the documentation for the project. Note that for this use case, ideally it would be even better to add XWiki Syntax 2.x support to Maven Doxia but that's another story.