Allow per-format options

[mwouts]

Jupytext allows multiple text representations for a single file, but the options are global on the notebook, while ideally they would apply to a given representation.

The purpose of this issue is to

• Better define the concept of an option on the format (split_at_heading, cf. Split Markdown cell at heading by default? #130, could be an option for markdown representations)
• Allow different options for the various representations (cf. below a possible example with two variants of a markdown representation)
• Allow default options for each format
• Give a larger degree of freedom on the file name for a given representation, cf. File names with dot '.' #138 or Jupytext plugin for the pre-commit package, compatible with black and flake8 #142 (python representation in a subfolder)
• Structure the options in a way that is compatible with a Jupyter extension Jupyter extension for Jupytext #86

[mwouts]

I am considering adding a new entry in the metadata, formats_options. A sample metadata may look like this:

jupytext:
  formats: ipynb,py:percent,md1,md2
  formats_options:
    md1:
      extension: .md
      format_name: markdown
      metadata_filter_cells: -all
      prefix: ../markdown/
    md2:
      extension: .md
      format_name: markdown
      metadata_filter_notebook: -all
      prefix: ../markdown_split_at_heading/
      split_at_heading: true
    py:percent:
      comment_magics: false
      extension: .pct.py
      prefix: ../python/
  text_representation:
    format: md2
    format_name: markdown
    format_version: '1.0'
    jupytext_version: 0.9.0

[mwouts]

In version 1.0, the main jupytext functions (read, write...) use a fmt argument that is either the usual format string (e.g. py:percent), or a dictionnary that can receive format options. However, options in jupytext metadata remain global and shared among all text representations.