Support conditional transitions

[PWAlessi]

In the Javascript implementation, the EventDesc Dst (To in the JS implementation) can accept a function to implement conditional transitions (https://github.com/jakesgordon/javascript-state-machine/blob/master/docs/states-and-transitions.md#conditional-transitions).

FSM should allow conditional transitions with the target state determined via a function call at runtime.

[maxekman]

Will evaluate this at some point, thanks for suggesting.

[jjauzion]

Any news on this one ?

[maxekman]

I’m currently not using FSM myself, but I’m open to review a PR with an implementation and take it from there.