ASCIIDOCTOR formatter: suppress embedded HTML

By default the ASCIIDOCTOR formatter now suppresses HTML that is
embedded in asciidoc by using '++++' blocks. Administrators can allow
embedded HTML by setting formatter.ASCIIDOCTOR.allowHtml to true in
the plugin configuration file, but allowing HTML is a security risk.

Change-Id: Idb283e6f93c543aca7944476dae954204a5bce23
Signed-off-by: Edwin Kempin <edwin.kempin@sap.com>