initial commit of Asciidoctor backend

Here's the very start of a packt backend for use with Asciidoctor. I choose to use the slim template languages because it's the most concise language for creating XML output.

I've just done (most of) the document template so far. This covers the [header] and [footer] parts of the AsciiDoc Python backend. The next step is to do the paragraph block, then enable call to `=content` in the document template. Once you have these started, the rest should come quickly.