Prevent over-escaping in valid emoji shorthand

[yhatt]

Resolves #98.

<Escape> component and <i> intrinsic HTML tag will detect valid emoji shorthands (includes custom emoji) and leave underscore to render emoji correctly.

<Blocks>
  <Section>
    <p>_foo:arrow_down:bar_</p>
    <p><i>foo:arrow_down:bar</i></p>
    <p><Escape>_foo:arrow_down:bar_</Escape></p>
  </Section>
</Blocks>

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