Optionally Hide Devices

While we work on more accurate device detection in #17, it might be worth it to allow customizing individual devices based on Friendly Name or IEEE address. This change would create a structure like the following in config.yaml:

```
devices:
0xdeadbeefdeadbeef:
hidden: true
```

I also propose we add a way to explicitly add devices as opposed to explicitly hiding them via a global config setting like:
```
bridge:
...
devices_implicitly_hidden: true
```

Which would set the default device hidden value to `true` as opposed to `false`, and allow for explicit visibility in the `devices` region. Let me know what you think, and thanks again!