Look into supporting chained exceptions

We're adding support for chained exceptions in Elastic APM (see elastic/apm#40).

However, we don't have a way to chain in exceptions in Node.js, but I was thinking we might provide an API that allows the user to record such things.

E.g. it's not uncommon in Node.js to say get a low-level error back from the database driver and then throw a more high-level error. Having access to both the low and the high-level error would be useful.