Add support for simplesect DocBook element

[fbolton]

Please add support for the simplesect DocBook element (which is something we use a lot in the JBoss Fuse library). The simplesect element can be processed almost identically to a regular section element. But I would like the converted simplesect to be annotated with metadata to ensure that it can be converted back into a simplesect element when it is processed by ccutil. In other words, I would like the following DocBook sample:

<simplesect xml:id="Sample-ID">
  <title>Sample Heading</title>
  ...
</simplesect>

To convert to:

[simplesect]
[[Sample-ID]]
===== Sample Heading

Where the number of equal signs above obviously depends on the current level of indentation. I tested this AsciiDoc snippet and found that it gives back exactly the original DocBook shown above, when you convert it using asciidoctor -b docbook <filename> (which is what ccutil does when building the book). The big advantage of this (from our perspective) is that the Red Hat docs toolchain currently supports simplesect in its CSS stylesheets, so our books will look much nicer if we preserve this metadata.