Added the ability to delete keys to WriteStream.

The LevelUP API does not include a way to stream deletes. This pull-request fixes that, by adding an option to the WriteStream, like so:
```
db.createWriteStream({ type: 'del' })
```

I added this because having streaming deletes might simplify the implementation of them in https://github.com/mcollina/node-levelgraph.

I also added some docs to the README.md for this flag.