display binary data as strings, not objects

```js
db.put('foo', new Buffer('bar'));
```

```bash
$ lev ./db --keyEncoding utf8 --valueEncoding binary --get foo

bar
```

I didn't go for hexdumps as I want the string representation. Maybe we could go for

```js
<62 61 72> bar
```

but that looks ugly with big buffers.

Fixes #8.