Add support for 'include' directive in reStructuredText preview in source browser (BB-8854)

It would be great if the include directive worked in the source browser.

The documentation for it is at http://docutils.sourceforge.net/docs/ref/rst/directives.html#including-an-external-document-fragment and an example of where I would like it to work is at https://bitbucket.org/volumesoffun/polyvox/src/master/documentation/changelog.rst