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).
No, I mean:
if I define a new custom log level `foo` this.graylog2 would not have such  a method and would throw and exception?