This PR implements a new flag allowDuplicates in FlutterBlue.scan method.
The flag allows to specify whether or not the scan stream should continuously emit devices that were previously discovered.
This is equivalent to set the CBCentralManagerScanOptionAllowDuplicatesKey flag in iOS CBCentralManager and it is useful in those scenarios where you want to keep monitoring RSSI values during the scan phase.
This would also solve #35

An example of the intended use would be:

var _fblue = FlutterBlue.instance;
_fblue.scan(
  allowDuplicates: false,
  withServices: [
    Guid("00001826-0000-1000-8000-00805f9b34fb")
  ]
).listen((scanResult) {
  // ...
});

Please note that the default value of the flag is set to false, which is the most common use case of a BT scan procedure.