Incorrectly defaults to strict mode, no escape hatch

I need to use `with` in a readme, and evalmd chokes on it because it defaults to strict mode (which JS does not do, and so neither should evalmd).

If changing that default is a problem, then there'd need to be some kind of way to get out of strict mode into sloppy mode - perhaps an HTML comment before the code block, or an argument passed to the binary?