Allow memoization of function parameter

[hc-12]

This would allow transformer to accept function as prop which may have custom properties.

[coveralls]

Coverage increased (+0.3%) to 92.778% when pulling addc4eb on hc-12:patch-2 into 5839f69 on mobxjs:master.

[mweststrate]

@hc-12 curious about the use case.

Also, could you add a unit test verifying the change?

[hc-12]

@mweststrate I needed to pass in function that has a prop that i need to check against the observable state.

filteredProp = createTransformer(fn => {
   return this.state.filter(prop => prop === fn.propName);
});

Is there any reason to limit other types besides from null & undefined?

I've updated PR to include test cases. Let me know if any change is required.

[mweststrate]

Released as 5.1.0