Making standalone optional

[wcaleb]

Is there a particular reason why Docverter hard codes the Pandoc standalone option for all output? I understand why it would be needed for PDF/DOCX/EPUB output, but for other output formats it might be useful to be able to convert snippets without returning full document headers.

This occurs to me primarily because I have written a Docverter workflow for Editorial on iOS, and it would be nice to be able to select text in Editorial, convert, and replace that selected text with the output text, without all the full document headers.