Adds `mkdirp` to  create output directories for Pandoc
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When using the `-o` option from Pandoc, the output directories have to exist. This means, if for example `pandoc -o <someDir/output.pdf> <input.md>` is used, the directory `someDir` must already exist.

To ensure this, the function `mkdirp.sync` is used before passing the modified options to spawn Pandoc. The reason why `mkdirp.sync` is used instead of `fs.mkdirSync` is because the ability to create directories recursively is needed.