AsyncCompleter for readline

Previous Completer interface required implementation
of BOTH types (sync and async). Sync type could
be assigned to async type, but not vice versa, forcing
implementers to always implement the sync interface.

node/node-tests.ts

@@ -1265,6 +1265,12 @@ namespace readline_tests {
                 return [['test'], 'test'];
             }
         });
+        result = readline.createInterface({
+            input: input,
+            completer: function(str: string, callback: (err: any, result: readline.CompleterResult) => void): any {
+                callback(null, [['test'], 'test']);
+            }
+        });
     }
 
     {