Support custom yamux.Config

When using the dlv debugger, the plugin's RPC call will be terminated while the debugger is paused. I have gotten around this by making this change in

https://github.com/hashicorp/go-plugin/blob/a02e5c7a911db823ae167d00bc1024f99d340258/rpc_client.go#L63

```diff
- mux, err := yamux.Client(conn, nil)
+ sessionConfig := yamux.DefaultConfig()
+ sessionConfig.EnableKeepAlive = true
+ sessionConfig.ConnectionWriteTimeout = time.Minute * 5
+ mux, err := yamux.Client(conn, sessionConfig)
```

Is it possible to allow for a config like this, or an alternative way to avoid having the RPC connection be interrupted?