add ruby annotation support

[MichaelCharles]

DenDen Markdown is a popular implementation of ruby annotations in markdown, and a markdown-it extension already exists for it. By installing markdown-it-ruby the following markdown,

{電子出版|でんししゅっぱん}を手軽に

will be converted into this HTML,

<p><ruby>電子出版<rt>でんししゅっぱん</rt></ruby>を手軽に</p>

Screenshots

Before

After

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solves #1830