Implement 'active' option for py and R scripts

[mwouts]

Implementation should allow round trip conversion of cells that are not active in scripts. For instance cells with jupyter magic (another workaround for that case being #29 ).

[mwouts]

Also required for #32

[danieltomasz]

I dont know where to left it, but this is interesting for R scripts https://webbedfeet.netlify.com/post/interchanging-rmarkdown-and-spinnable-r/

[mwouts]

Hello @danieltomasz , thanks for the reference. The spin format for R scripts in Jupytext is intended to be compatible with knitr's spin function, cf. this test. The jupytext commands for converting R Markdown to R spin are therefore:

jupytext --to Rmd script.R
jupytext --to R notebook.Rmd

Please let me know (i.e. open an issue) if you spot imperfections in the conversion.