Add the ability to specify `ready` for individual integrations

[calvinfo]

Something like:

analytics.ready('Mixpanel', function(){
  mixpanel.alias('foo');
});

so that users can have better control over which integrations they actually need to be ready in case one doesn't load.

[hadees]

+1 we've had some integrations that failed for some reason blocking all the other ones.

[kanoshin]

+1, it is sad, that this method is called only after ALL integrations are loaded. If I turn off some of them with page method, i still want to understand when other integrations are ready..

[dwkoogt]

+1

[mstockm]

+1

[TimeBomb]

+1
AdBlock breaks analytics.ready because we use the AdRoll integration. We have to create a custom interval to check when MixPanel is loaded, because we need to call mixpanel.set_config.