--feature_mask

Zhen has implemented an option to specify --feature_mask that limits the features which may be updated during training.

One could use it for training a model with --l1 regularization and then retraining without regularization using just the features that survived:

```
vw --l1 0.001 -f model.l1
vw -i model.l1 --feature_mask model.l1 -f model.final
```

Some of the bitmask loading needs to be refactored but wanted to send this to you so that we can discuss Friday.