Implicit includes

[majkinetor]

It would be great to add another option, list of files that are included implicitelly.

Those would serve to include headers and footers for each markdown.

Config could look something like:

configs = {
            'mdx_include': {
                'footer': 'some footer text', 
                'header': |

                          ---
                          {!header.md!}
            },
        }

Alternativelly, there could be a convention - if file in root of the default location is named _header.md or _footer.md it gets included in every markdown.