Add ability to process other document extensions

[SimonCropp]

defined by DocumentExtensions in config https://github.com/SimonCropp/MarkdownSnippets/blob/master/docs/config-file.md

[claremacrae]

I read the docs from this - and after two readings, I still can't work out what the feature is.

It says "process" other docs - but what kind of processing? A little more description to say what it is, and why one might use it. would be helpful...

[SimonCropp]

@claremacrae @jawn does this answer your question https://github.com/SimonCropp/MarkdownSnippets#documentextensions ?