support optional elements and attributes

[cornerman]

This is my attempt to fix #22. Does it make sense to you?

[LukaJCB]

LGTM, thank you very much :)

[PerWiklander]

Nice. Thanks!
Did we really need the new operator :=? though?

[cornerman]

In the corresponding issue I have looked at the link from scalajs-react and implemented it with the same operator, which makes the semantics explicit. Overloading the := operator would also be fine with me; but trying it now, I am also not really able to convince the compiler to use the option overload :)

[PerWiklander]

Being explicit might be a good idea actually. But... wouldn't having OptionIsEmptyModifier in scope make it work with := ? So that I could choose to import that if I want to use the normal operator and be implicit?

[cornerman]

Well, the implicit is already in scope, you can already write it like so (which is the same as :=?):

input(
  Option("bar").map(data.foo := _),
  Option.empty.map(data.bar := _)
)

However, I also tried importing the implicit but did not help. Ideas welcome!

[PerWiklander]

Sorry, I slipped while thinking :-)