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).
Agreed on extending. Just another case comes to mind. What if people supply an invalid levelMap. like {info:"someNonGraylog2Level"}. Should we have a check?