runctl: add heapdump command

Depends on https://github.com/strongloop/node-heapdump/pull/1
I hate the name :-(. It doesn't match the "object-verb" pattern of the other profilers... "heap-dump" would, but I wonder if we could name the command after the chrome extension, and call this "Snapshot heap objects...", and the command "heap-snapshot". Thoughts? Maybe you can schedule a mini-demo of this (should just be 5 minutes) and we can bike-shed the name. :-)