Implement mixer shutdown

Author: dsc8x

README.md

@@ -35,6 +35,7 @@ Currently this client can:
 * activate PAFL for each channel
 * set the fader level for each channel
 * activate the mute button for each channel
+* shut down the mixer
 
 If you need more features then feel free to open a GitHub issue and I will add them.   
 
@@ -95,6 +96,10 @@ mixer.setMute({
   active: false
 })
 
+// Shut down
+mixer.shutdown() 
+
+// Release connection to mixer
 mixer.disconnect()
 ```
 

src/client.js

@@ -50,6 +50,14 @@ class Client {
     output.sendMessage([0x80, channel, 0x00])
   }
 
+  shutdown() {
+    this.setNrpnParameter({
+      channel: 0,
+      cmd: 0x5f,
+      valueIndex: 0,
+    })
+  }
+
   setNrpnParameter({ channel, cmd, value, valueIndex = 0x07 }) {
     const { output } = this
     output.sendMessage([0xb0, 0x63, channel])