Allow iframe and embed tags

[Peter Fleck <peterfleck@gmail.com>]

Submitted on Github as #95 - https://github.com/docbook/docbook/issues/95

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Propose adding iframe and embed tags.

My specific use case is a video datasource that only allows iframe 
embedding, using Docbook's mediaobject/videoobject doesn't work.

Maybe have some kind of correlation with what schema.org have done with 
mediaobject -
 http://schema.org/MediaObject