Add support for quote and verbatim

[rytswd]

Fixes #40

Added

• [Markdown] Quote support to prepend all lines with > with a new option style: quote
• [Markdown] Verbatim support for wrapping imported contents with "```", with a new option of style: verbatim yaml for YAML, or `wrap: yaml` for short

Fixed

• [Markdown] Import with Exporter Marker is now supported against YAML file

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Other Notes

Exporter Markers were originally meant to be used for the same file type, e.g. YAML to import another YAML. But this can be probably useful to work across file types. As Markdown was clearly the one that needed this support first, I got a fix for it, but I'll need to check other file types and ensure this pattern is supported. It would require much more refactoring, though, and thus I kept such updates minimal for this PR.