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 should probably go in the jsdoc rather that in the middle of the code :) but we should probably prevent people from passing an empty levelMap or, better, merge it with the standard one so that one can override / add its own levels