Version numbering for text-only notebook formats

[mwouts]

Text only notebook formats have a priority over ipynb notebooks. This may cause users to have their notebooks changed due to a file format update in nbrmd.

When both py and ipynb files are present, nbrmd should refuse to load py files that were not generated with the current version format, and instead, invite the reader to remove the py file (and get it regenerated in the new version, from the ipynb file).

[mwouts]

The commit reference above will refuse to open a notebook from a pair of py, ipynb file when the py file has a version other than the current version, and display a message like this one

File Untitled1.py has nbrmd_file_format_version=None, but current version for that extension is 1.0. It would not be safe to override the source of Untitled1.ipynb with that file. Please remove one or the other file.