Lua Internals: replace standard Lua Patterns with `lpeg` grammars

[cscheid]

We use Lua patterns everywhere, but pandoc ships with lpeg and (lpeg.re) specifically. We should definitely switch to those.

(cc @cderv: lpeg is a great parsing engine, it turns out, and lpeg.re is a regex engine with | inside it :))

• Lua Patterns
• lpeg
• lpeg.re