preEmit and shouldEmit in actions

[krawaller]

Do we really need both preEmit and shouldEmit? Code from preEmit could just as easily be put inside shouldEmit instead.

I see the values of the explanatory names and not having to actively return true if you just want to run code pre-emission, but is that worth muddying the API?

As for having to return true to proceed, we could switch it so that emission is cancelled if return value is true.

[spoike]

There is currently a little implementation mistake with preEmit... it really should take a return value and emit that value as argument instead. If undefined it should just pass through the arguments as the action got it.

See #58

[krawaller]

Excellent discussion there, didn't think to search first, sorry!

And allowing preEmit to morph the data (clever idea!) of course justifies the existence of shouldEmit.