Add support for using various extensions for the source test file.
Add support for using various extensions for the source test file.

This lets you use non *.html extensions for your source file. For example,
files can have file extensions such as *.text *.md but are all converted to
a file with an extension *.html, so that websites can readily push ages., and the
site generation to output a standard *.html extension.

e.g. red.md --> becomes red.html for internal processing

mhils/countershape,samtaufa/countershape,cortesi/countershape,mhils/countershape,cortesi/countershape,samtaufa/countershape