Path for custom Pandoc templates / select template on export

[StuartJMackintosh]

Pandoc uses a Latex template to create files from (see /usr/share/pandoc/data/templates/).
A template can be exported (ie pandoc -D latex >template-local.latex) and then locally modified.

Exposing a separate volume where pre-existing modified templates can be saved, then enabling the user to select a template associated with the export type enables the user to access their custom designs straight from CodiMD.

Without this, the only way to process an external template is to use the CodiMD /download link, pipe through external Pandoc with custom command.