Shiny: Add support for ifelse logic in conditionalPanel

[daattali]

Many times I want to do something along the lines of

conditionalPanel("input.test", "yep"),
conditionalPanel("!input.test", "nope")

Some syntactic sugar here would be awesome :) Hopefully the above could be translated into

conditionalPanel("input.test", iftrue = "yep", iffalse = "nope")

(clearly I'm not great at naming arguments)

[daattali]

@jcheng5 I keep running into cases where I really want this functionality.

What does the shiny team think about

conditionalPanel(condition, ..., ns = NS(NULL), else = NULL)

I used to be absolutely terrified about this idea, but now that conditionalPanel already has another named argument, I'm starting to think it's not that bad.

[alandipert]

Sorry for taking so long to get to this 😄 This is a nice enhancement, I think it's safe to say we'd consider a PR.