Add support for Markdown safe_mode on input text.

This change is driven by:

    https://github.com/cdent/tank/issues/61

A new config setting 'markdown.safe_mode' is available. It defaults
to 'escape'. The other options are described in the Markdown docs:

    http://pythonhosted.org/Markdown/reference.html#safe_mode
release 1.2.4