[Experimental] --pptx-editable option to convert Markdown into editable PPTX

[yhatt]

Resolves #166 and #298.

This PR is adding an experimental --pptx-editable option to convert Markdown into editable PPTX when converting with --pptx option. The output can open in common presentation softwares with keeping reproducibility in most of slides, and can edit the text, image, and so on.

marp --pptx --pptx-editable slide.md

We have described how to make editable PPTX from Marp Markdown in marp-team/marp#82, but this solution is depending on the commercial software (Adobe Acrobat) or non OSS solution such as PDF to PPTX web services. I had thought that there was nothing the solution for making editable PPTX by OSS, but I've found LibreOffice has a conversion feature with headless soffice --headless, that can use the PDF input filter and the PPTX output filter.

By this implementation, we can provide completely open-source projects based solution to convert HTML-based slide into editable PPTX, without uploading a draft slide into third-party web service.

This implementation will convert Markdown into PDF first, and try converting that into PPTX with the headless LibreOffice. Thus, users have to install LibreOffice before using --pptx-editable option.

Limitations

• The slide reproducibility would become lacked rather than existing PPTX conversion. You should not expect the perfect reproducibility of the slide preview in the output PPTX.
• If the rich design has been included to the slide, such as using gaia, uncover built-in theme and custom themes, may break the layout or fail conversion due to that cannot load with LibreOffice.
• Unlike the current PPTX conversion, the speaker notes (<!-- notes -->) are not included in the output PPTX.

The almost part of this conversion process is depending on upstream tools, the browser (HTML -> PDF) and LibreOffice (PDF -> PPTX), so there is no room to control intermediate outputs by Marp. So it is difficult to fix these limitations in Marp.

Due to that limitations, we are planning to ship this feature as an experimental feature.