Support source file including

[gaborantal]

This patch allows to automatically format source files (declared in the 'extensions' config option, where one can set a source file extension and a syntax highlighter synonym for that extension). I use it in mkdocs, where i set the following extensions:

    - markdown_include.include:
        base_path: docs
        extensions: 
            java: java
            py: python
            js: javascript

Is there any chence for this to get merged? Or do you plan to support this independently from my work?