support remote logging

[pq]

Related to #21, @matthewtsmith and @johnpryan described a desire to log remotely.

Use case: would like to redirect logging output to firebase to capture debugging data from devices in the wild.

https://github.com/SwiftyBeaver/SwiftyBeaver came up as a reference point.

I think for us to support this we just need something as simple as a listener interface a la package:logging where you can do something like:

Logger.root.level = Level.ALL;
Logger.root.onRecord.listen((record) {
  // redirect here...
});

/cc @devoncarew