Support files inclusion without a block

[Weng Shiwei 翁士伟]

The feature of file inclusion is cool, but I wonder is it possible to include a file without a block

<!-- $MDX file=my_file.md -->

This will render to (''' are three backticks. I don't figure out how to escape it in github code block)

<!-- $MDX file=my_file.md -->
'''
CONTENT_OF_MY_FILE
'''

I hope to have

<!-- $MDX file=my_file.md -->
CONTENT_OF_MY_FILE

It can be used to split one giant markdown to multiple files.