Treat emoji seriously

[cscheid]

We have passable emoji coverage in our pandoc_str regex, but we should figure out how to get https://github.com/mathiasbynens/emoji-regex integration.

[cscheid]

Here's a fundamental problem: *️⃣ starts with *, and so it will interfere with the architecture of the parser and external lexer. People will be able to write `*️⃣`{<markdown} or `*️⃣`{=pandoc-str}, but we will not be able to directly suport this one particular emoji (and I suspect a few other emojis might be problematic as well).

EDIT: From https://www.unicode.org/Public/emoji/latest/emoji-sequences.txt, it seems that *️⃣ is the only emoji that is impossible for us to integrate.