Expose Flightplan constructor

[th3l0g4n]

When requiering flightplan alsways the same instance will be returned.

It would be very helpful if in https://github.com/pstadler/flightplan/blob/master/lib/index.js, maybe additionally to the instance, the actual constructor was exported to create new instances an demand.
This would making embedding flightplan into other applications much more easier.

[pstadler]

Definitely thinking about adding that.