Allow objectMode to be overridden

This allows me to do something like:

```javascript
Dissolve({'objectMode':false}).loop(function (end) {
  this.uint32le('len')
  .buffer('data', 'len')
  .tap(function () {
    this.push(this.vars.data)
  })
})
```

I can then `pipe()` that into some other stream.