add REPL config, help, and document

Provide ability to configure REPL via config.

* `repl` - where to have REPL listen, defaults to `env.CLUSTER_MASTER_REPL` || 'cluster-master-socket'
  * if `repl` is null or false - REPL is disabled and will not be started
  * if `repl` is string path - REPL will listen on unix domain socket to this path
  * if `repl` is an integer port - REPL will listen on TCP 0.0.0.0:port
  * if `repl` is an object with `address` and `port`, then REPL will listen on TCP address:PORT

Examples of configuring `repl`

```javascript
var config = { repl: false }                       // disable REPL
var config = { repl: '/tmp/cluster-master-sock' }  // unix domain socket
var config = { repl: 3001 }                        // tcp socket 0.0.0.0:3001
var config = { repl: { address: '127.0.0.1', port: 3002 }}  // tcp 127.0.0.1:3002
```

Provide help command within the REPL

Document REPL use