fixup convert metadata
add `nbformat.writes(version=X)` for downgrade

allows easy downgrade of notebooks.
support downgrading notebooks with nbconvert

Use:

    ipython nbconvert --to notebook --nbformat 3 notebook.ipynb

to write `notebook.v3.ipynb`

Adds `output_suffix` resource, inserted before file ext.
Suffix is used if `--output` is not specified.

Default is `.nbconvert`, to prevent `--to notebook` from clobbering existing notebooks by default.