Make moment a weak dependency

[jankapunkt]

I am currently using autoform without the need for any of the momentjs related functionality. Hower, I know that a) some tests rely on some functionality that is really hard to replace when removing moment and b) many projects may have still quite a dependency on moment.

On the other side, the moment package may have evolved already and we may be able to use only parts of it, instead of the whole 60KB.

Anyone interested in making moment a weak dependency?