Decouple language format

[GCBallesteros]

This is a bit more radical than my previous pull request. It completely decouples notebook language and format and eliminates the need for an additional global setting. In this PR the format is extracted from g:jupytext_fmt if available. It also eliminates the need to hardcode the language/format combinations in the plugin.

I've tried to make it backwards compatible so current configurations are not affected. You can still use

let g:jupytext_filetype_map = {"py": "r"} " silly example

to map languages to arbitrary buffer filetypes if required.

I believe this is a better solution to the language/format combination problem than the one presented in my other pull request but it would require a more thorough testing to assure that it doesn't break things for current users.