Initial version of oembed support

- link to qoembed library
- add new class OEmbedWorker that retrieves the HTML code
  for a rich media link
- modify HtmlPreviewGenerator to use the new OEmbedWorker class
New classes TextPreprocessor and HtmlPostprocessor

This classes will be called by the MarkdownConverter implementations.

The class TextPreprocessor will be called before a MarkdownDocument is
created from the text.

The class HtmlPostprocessor will be called after the HTML is generated.
TextPreprocessor & HtmlPostprocessor in MarkdownConverter

Added possibility to add a TextPreprocessor and a HtmlPostprocessor to the
MarkdownConverter class.

Adapt the discount and hoedown implementation to the changed interface.
EmbeddedMediaPreprocessor/EmbeddedMediaPostprocessor

Add new classes EmbeddedMediaPreprocessor and EmbeddedMediaPostprocessor.

The preprocessor turns embedded media links @[]() into HTML links with
class "embed".

The postprocessor retrieves the media resource for the HTML links with
class "embed" and replaces the link with the HTML code for the resource.
Changed embedded media support

Replaced OEmbedWorker and HtmlPreviewGenerator::preprocessMarkdown() with
the new EmbeddedMediaPreprocessor and EmbeddedMediaPostprocessor classes.