[feature] `isMaster` - True if the process is a master.

I am working on `forever start process.json`, now the problem is people wanna start apps in cluster-mode, but not only spawn/fork-mode, so, I need a value to indicate whether the process is a master or not.

- cluster-mode:
  To take advantage of multi-core systems the user will sometimes want to launch a cluster of Node processes to handle the load. The process is running in worker process (not master), it does not have an UNIQUE_NODE_ID.
- spawn/fork-mode:
  A single instance of Node runs in a single thread. The process is running in master process, it has an UNIQUE_NODE_ID.