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).
Please indent in your editor. When I tried to do auto indent in mine, git diff showed the entire file as changed, not sure why. That's why didn't indent.