Integrate with downdoc

[abelsromero]

User wants to be able to use downdoc from the plugin.
downdoc offers a way to export a Asciidoc to Markdown, which is in accordance with an AsciiDoc coverter.

For context: #618 (comment)

[mojavelinux]

It's important to note that this issue should be on hold until a Ruby-port of downdoc is available. That's something we are planning to offer in downdoc, but we're not saying when that will happen (as it depends on downdoc stabilizing).

[abelsromero]

but we're not saying when that will happen (as it depends on downdoc stabilizing).

No issue at all, marking as "tracking" so we get a reminder to check from time to time and maybe measure people's interest.