ReactiveCommand can set initial CanExecute state

ReactiveCommand's constructor can optionally set the initial CanExecute
state.  Originally, it defaulted to true, until your canExecute observer
told it otherwise.  It is of course possible to forced the state to
false before binding it to anything, but this removes that nescessity
Merge pull request #345 from sillyotter/reactive-command-initial-condition

ReactiveCommand can set initial CanExecute state