Convert the jupytext_formats metadata entry to jupytext/formats, etc. See #91

def rearrange_jupytext_metadata(metadata):
    """Convert the jupytext_formats metadata entry to jupytext/formats, etc. See #91"""

    # Backward compatibility with nbrmd
    for key in ['nbrmd_formats', 'nbrmd_format_version']:
        if key in metadata:
            metadata[key.replace('nbrmd', 'jupytext')] = metadata.pop(key)

    jupytext_metadata = metadata.pop('jupytext', {})

    if 'jupytext_formats' in metadata:
        jupytext_metadata['formats'] = metadata.pop('jupytext_formats')
    if 'jupytext_format_version' in metadata:
        jupytext_metadata['text_representation'] = {'format_version': metadata.pop('jupytext_format_version')}
    if 'main_language' in metadata:
        jupytext_metadata['main_language'] = metadata.pop('main_language')
    for entry in ['encoding', 'executable']:
        if entry in metadata:
            jupytext_metadata[entry] = metadata.pop(entry)

    filters = jupytext_metadata.pop('metadata_filter', {})
    if 'notebook' in filters:
        jupytext_metadata['notebook_metadata_filter'] = filters['notebook']
    if 'cells' in filters:
        jupytext_metadata['cell_metadata_filter'] = filters['cells']

    for filter_level in ['notebook_metadata_filter', 'cell_metadata_filter']:
        if filter_level in jupytext_metadata:
            jupytext_metadata[filter_level] = metadata_filter_as_string(jupytext_metadata[filter_level])

    if jupytext_metadata.get('text_representation', {}).get('jupytext_version', '').startswith('0.'):
        formats = jupytext_metadata.get('formats')
        if formats:
            jupytext_metadata['formats'] = ','.join(['.' + fmt if fmt.rfind('.') > 0 else fmt
                                                     for fmt in formats.split(',')])

    # auto to actual extension
    formats = jupytext_metadata.get('formats')
    if formats:
        jupytext_metadata['formats'] = short_form_multiple_formats(long_form_multiple_formats(formats, metadata))

    if jupytext_metadata:
        metadata['jupytext'] = jupytext_metadata