Implement Elmish commands in ReactiveCom

[EdouardM]

Hello @alfonsogarciacaro ,

I think it would be nice to implement Elmish commands in Fable.Helpers.React.ReactiveCom to make it fully compliant with Fable.Elmish.

I have played with this over the week-end and I am going to submit a PR to show what I came up with.

** Warning ** : This includes a new depedency to Fable.Elmish in the projet. If you want me to rewrite Elmish.Cmd<'Msg> I can do it. But I thought over time the codebase of the 2 project may not stay in sync.

Feel free to reply with suggestions.

Regards; Edouard