add createViewStream function

When reading map-reduced data from `createReadStream`, the keys that are returned are encoded, and don't correspond to the key that was originally emitted, making them hard to make sense of.

`createViewStream` returns the same stream as `createReadStream`, but emits a `row` event for each `data` event, and decodes the key.

Not sure if this is the "right" way of doing this, but it works. Let me know if there's a better way I should go about this.