Output transformations pipelines for wikitext

The classes in the Stages/ subdirectory contains HTML and DOM transforms for use in output processing pipelines, i.e. postprocessors for ParserOutput objects that either directly result from a parse or are fetched from ParserCache.

The default pipeline is created by DefaultOutputTransformFactory; it corresponds to what was previously contained in ParserOutput::getText. The shouldRun method in these stages uses defaults that indicates if the stage runs or not in the default OutputTransformPipeline.