Support for [!include]

[gl-yziquel]

It would be very nice if an [!include](other_markdown_file.md) syntax were supported.

It would allow me to write very nice readme.md files in a modular fashion in my repositories.

For instance, I could have TODO.md that would blow up in my face with frogmouth, but would be out of the way when editing the README.md that would include the TODO.md file. This would fit in nicely with my workflow.