runctl: add heapdump command

Depends on https://github.com/strongloop/node-heapdump/pull/1
I'm not sure heapdump is special enough to warrant breaking the nodeback convention (error or null as first argument). The exceptions I've seen are where the call itself could be described as an error check (eg. ENOENT in a file existence check is really not an error), but this looks like standard async.