Add Support for --id-prefix for Markdown

[patbakdev]

I have multiple markdown files that get 'rolled-up' into a single markdown file that eventually gets converted to html. I tried using the --id-prefix to make each lower level markdown file unique to avoid clashes when combined, but it doesn't seem to work for markdown. It would be great if it did.

For now, as a work-around, I pass in the prefix string as a metadata key and use a filter to update the indentifier.