Add lengthAdjustment option to LengthFieldPrepender

Because I want to write as follows:
```scala
ch.pipeline()
  .addLast("framer", new LengthFieldBasedFrameDecoder(17, 0, 1, 1, 1))
  .addLast("prepender", new LengthFieldPrepender(1, -1))
  .addLast("decoder", new FooDecoder())
  .addLast("encoder", new FooEncoder())
  .addLast("handler", new FooHandler())
```
This pipeline can communicate 1 to 16 bytes data.