Don't use JSON.parse/stringify to clone.

[drhayes]

Fixes #439.

Uses the clone npm package to provide a deep clone. This probably has performance implications since JSON.parse(JSON.stringify(thing)) is super fast, but I don't think the original implementation meant to throw away Date and Symbol objects.

[ianp]

You could call it as clone(message, true) to get better performance, because stringily/parse would fail with circular references in any case.

Other than that, this looks good to me to merge.

[benbrown]

Thanks @drhayes @ianp!