Denote optional, hidden inputs in custom blocks

[cycomachead]

Before we get to input groups, which would be amazing, a more simple start would be to allow users to specify text in an block spec, such as $optional, which would render the triangle and default to everything after the triangle being collapsed.

[brianharvey]

I would prefer not to make this a special case (although I'd settle for that, I guess), but instead have minimum/default/maximum number of slots settable for variadic inputs.

If we do make a special case for optional inputs, of course users shouldn't see a blockspec, but should instead see another radio button below the variadic one saying "optional input (be sure to provide default value above)."

[merlin2v]

that reminds me of my MultiMorph and CustomMultiMorph project where I was tring to create a mod that would create input groups using %( variadic inputs however getting it to function properly for the CustomMultiMorph had difficulties working properly. when I came back to use snap! I remembered that I had been trying to create a user interface type-of thing :) Here's the MultiMorph project that I completed: https://github.com/jmoenig/Snap/issues/1919 I probably learned the majority of what I know about js from trying to do this. Are you finally going to add input groups!?