Export Denote notes with Markdown

Exporting from Markdown requires an external processor (e.g., Markdown.pl, Pandoc, or MultiMarkdown). The markdown-command variable defines the command line used in export, for example:

(setq markdown-command "multimarkdown")

The export process thus occurs outside of Emacs. Users need to read the documentation of their preferred processor to prevent the creation of duplicate Denote identifiers.