Snoop filter by type

This PR really includes two changes:

* Printing out the IP address of the messages:

```
FROM: 192.168.11.33:56700
SIZE: 88
DATA: 000  58 00 00 54 00 00 00 00  d0 73 d5 00 0c af 00 00  |X..T.....s......|
      010  4c 49 46 58 56 32 00 00  00 00 00 00 00 00 00 00  |LIFXV2..........|
      020  6b 00 00 00 0a 16 32 33  ff ff ac 0d 00 00 ff ff  |k.....23........|
      030  f0 9f 94 ad 00 00 00 00  00 00 00 00 00 00 00 00  |................|
      040  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
      050  01 00 00 00 00 00 00 00                           |........|
MSG:  &{Version:1024 Target:[208 115 213 0 12 175 0 0] Site:[76 73 70 88 86 50] AtTime:0 Addressable:true Tagged:false Acknowledge:false}
      *payloads.LightState &{Color:{Hue:5642 Saturation:13106 Brightness:65535 Kelvin:3500} Dim:0 Power:65535 Label:🔭 Tags:1}
```

* Optionally, filtering by payload type:

``` sh-session
$ go run cli/lifx-snoop/lifx-snoop.go pan # where "pan" is the case-insensitive filter on payload name
FROM: 192.168.11.52:56700
SIZE: 36
DATA: 000  24 00 00 34 00 00 00 00  00 00 00 00 00 00 00 00  |$..4............|
      010  00 00 00 00 00 00 00 00  00 00 00 00 00 00 00 00  |................|
      020  02 00 00 00                                       |....|
MSG:  &{Version:1024 Target:[0 0 0 0 0 0 0 0] Site:[0 0 0 0 0 0] AtTime:0 Addressable:true Tagged:true Acknowledge:false}
      *payloads.DeviceGetPanGateway &{}

FROM: 192.168.11.33:56700
SIZE: 41
DATA: 000  29 00 00 54 00 00 00 00  d0 73 d5 00 03 c4 00 00  |)..T.....s......|
      010  4c 49 46 58 56 32 00 00  00 00 00 00 00 00 00 00  |LIFXV2..........|
      020  03 00 00 00 01 7c dd 00  00                       |.....|...|
MSG:  &{Version:1024 Target:[208 115 213 0 3 196 0 0] Site:[76 73 70 88 86 50] AtTime:0 Addressable:true Tagged:false Acknowledge:false}
      *payloads.DeviceStatePanGateway &{Service:1 Port:56700}
$ go run cli/lifx-snoop/lifx-snoop.go lightget
FROM: 192.168.11.52:56750
SIZE: 36
DATA: 000  24 00 00 34 00 00 00 00  00 00 00 00 00 00 00 00  |$..4............|
      010  4c 49 46 58 56 32 00 00  00 00 00 00 00 00 00 00  |LIFXV2..........|
      020  65 00 00 00                                       |e...|
MSG:  &{Version:1024 Target:[0 0 0 0 0 0 0 0] Site:[76 73 70 88 86 50] AtTime:0 Addressable:true Tagged:true Acknowledge:false}
      *payloads.LightGet &{}

FROM: 192.168.11.52:56750
SIZE: 36
DATA: 000  24 00 00 34 00 00 00 00  00 00 00 00 00 00 00 00  |$..4............|
      010  4c 49 46 58 56 32 00 00  00 00 00 00 00 00 00 00  |LIFXV2..........|
      020  65 00 00 00                                       |e...|
MSG:  &{Version:1024 Target:[0 0 0 0 0 0 0 0] Site:[76 73 70 88 86 50] AtTime:0 Addressable:true Tagged:true Acknowledge:false}
      *payloads.LightGet &{}
```