Default pandoc input format is inconsistent with VS Code's renderer

[arthurward]

The VS Code previewer targets CommonMark[1], but the call to Pandoc lets Pandoc guess that the input is Pandoc's extended Markdown, which is slightly different. I added "-f commonmark" to the pandoc.(docx,html,pdf)OptString settings to force it to use CommonMark, which works. It seems to me that the default behavior should match what the VS Code preview window uses.

[1] See "Common Questions" at the end of https://code.visualstudio.com/Docs/languages/markdown