allow _tmpDir to be configured

dependent on: https://github.com/broccolijs/broccoli/pull/224
Can you make this a function?  Just something simple like:

```javascript
Funnel.prototype.getTmpDirPath = function() {
  var tmpRoot = this.tmpRoot || 'tmp';
  var randomPath = path.join(tmpRoot, 'funnel-dest_' + generateRandomString(6) + '.tmp');

  return path.resolve(randomPath);
};
```

Once ^^ is done, I'd like to have a few simple unit tests confirming the various edge cases work properly (if `this.tmpRoot` is and is not specified, that a `path.resolve` was called, etc).