Add a "next" property to help RxCommand interop with Futures

`next` is a new property which lets us await on the next invocation of
the command. This is nice on its own, as well as making it easier to
chain RxCommands together.

Fixes fluttercommunity#16

lib/rx_command.dart

@@ -200,6 +200,9 @@ abstract class RxCommand<TParam, TResult> extends Observable<TResult> {
   /// If no subscription exists the Exception will be rethrown
   Observable<dynamic> get thrownExceptions => _thrownExceptionsSubject;
 
+  /// This property is a utility which allows us to chain RxCommands together.
+  Future<TResult> get next => Observable.merge([this, this.thrownExceptions.cast<TResult>()]).take(1).last;
+
   Subject<CommandResult<TResult>> _commandResultsSubject;
   Subject<TResult> _resultsSubject;
   final BehaviorSubject<bool> _isExecutingSubject = new BehaviorSubject<bool>();

test/rx_command_test.dart

@@ -415,6 +415,35 @@ void main() {
     expect(command.isExecuting, emits(false));
   });
 
+  test("async function should be next'able", () async {
+    final cmd = RxCommand.createAsync((_) async {
+      await Future.delayed(Duration(milliseconds: 1));
+      return 42;
+    });
+
+    cmd.execute();
+    final result = await cmd.next;
+
+    expect(result, 42);
+  });
+
+  test("async functions that throw should be next'able", () async {
+    final cmd = RxCommand.createAsync((_) async {
+      await Future.delayed(Duration(milliseconds: 1));
+      throw Exception("oh no");
+    });
+
+    cmd.execute();
+    var didntThrow = true;
+    try {
+      await cmd.next;
+    } catch (e) {
+      didntThrow = false;
+    }
+
+    expect(didntThrow, false);
+  });
+
   Stream<int> testProvider(int i) async* {
     yield i;
     yield i + 1;