Added support for custom level mapping.

User can provide a levelMap object in options object passed to the contructor(exported as Graylog2). This will be used to map winston levels to graylog2 levels, instead of the default mapper. If a level has no mapping then it will be mapped to level 'info' (same as before).
I think this is really not formatted well, look at the final 2 lines:

```
}
}
```

I think you have some weird problems while indenting or writing...