support sending to multiple different certs

Hi,

At the moment it appears I can configure exactly one Android API key and one IOS push certificate.

I would like the capability to somehow disptach to different Android API keys and IOS push certificates, perhaps based on an app bundle id. I realize I could run N instances of gorush to accomplish that but it seems more ideal to be be able to do it all from the same process.

What do you think?

e.g.

```
android:
com.foo.bar:
enabled: true
apikey: "YOUR_API_KEY1"
max_retry: 0 # resend fail notification, default value zero is disabled
com.blah.blah:
enabled: true
apikey: "YOUR_API_KEY2"
max_retry: 0 # resend fail notification, default value zero is disabled
```