Add encode.bytesWritten

This PR adds `encode.bytesWritten` which is similar to `decode.bytesRead`.
This is useful is you pass the data array to know how big the varint written was:

``` js
var buffer = new Buffer(1024);
var offset = 0;
varint.encode(42, buffer, offset);
offset += varint.encode.bytesWritten;
```

I can add tests/docs also if you want.