Lint documentation examples #14

I've had a couple of issues where examples in documentation comments have been incorrect, due to using non-existent variables, etc...

While we do ensure examples are syntactically valid Lua, we don't run any other linters. We probably should do so. We want a way to opt out of linting (though probably not the syntax check). The easiest way would probably be to allow writing lua :nolint as the language type.

One further extension would be to allow arbitrary configs in the linter (for instance, lua :illuaminate:(linters (-var:unused-arg))), but that might be a little too much.