Handle Raw NB Cells

[chrisjsewell]

Currently, these cells are just ignored. We should maybe assess nb.metadata.raw_mimetype, which will be e.g. text/html, then do the same as the only directive does, to only output it given a matching builder

[amichuda]

In regards to the my closed issue #295 , how exactly would it work? Would I add something like :tag: raw in the code cell or something? Or would this be through glue? Trying to write my thesis in jupyterbook, so this would be a super useful feature!