Refactored the code of all the built-in plugins

  * Complex methods have been splitted into several methods
  * Generic method names (onItemPreParse()) have been replaced
    by specific method names (fixHtmlCode(), addLabels(), etc.)
  * All methods are now documented
  * Some code has been tweaked to make it easier to understand
  * The performance of the application remains the same (checked
    with the ./book benchmark command)