add .destroy method

Similarly to the old through module (and many other stream modules) this adds a `.destroy()` method that emits close on the next tick.

This allow you to easier add teardown logic

``` js
var makeStream = function() {
  var stream = through2(...)
  stream.on('close', function() {
    // in case stream was destroyed cleanup
  })
  return stream
}

var s = makeStream()
...
s.destroy() // something went bad - destroy the stream
```
@rvagg wasn't really a reason for that to be public. i've changed to it be to `._destroyed` now