[Feature Request] Allow use of a custom PDF generator

I'm not really happy with the way the browser engines render PDFs from the markdown files. Code blocks get split up in pages, sometimes lines are split in the middle, etc. So I've created a custom LaTeX template for pandoc to do these conversions, instead of docfx. Nevertheless, docfx is great and provides a nice solution for .NET project documentation.

I'd love to be able to change the default executor for the PDF conversion from Chromium to a custom command, that takes specified arguments and builds a PDF that docfx then integrates seamlessly.

Currently, I've implemented a hacky solution, where I explictly link the PDF files in the tocs as child items, and call pandoc before running docfx. It works, but it's clunky.