Insert into contents.lr rather than clobber

[obestwalter]

problem: I want to keep my data in contents. lr

solution: if the plugin finds a section wrapped in special comments, the markdown will be inserted in that section e.g.:

<!--JUPYTER_PREPROCESS-->
content from a jupyter notebook

...

lots of it
<!--JUPYTER_PREPROCESS-->

[obestwalter]

See if generating attachment(s) + embed-x might be a good idea.