Optional attribute support

In some library/wrapper code we need possibility to add attribute value conditionally. Something like this will be really helpfully:

  "OptionAttribute" in {
    val width = Var(scala.Option.empty[Int])

    @dom val td = <td width={width.bind}/>
    td.watch()

    assert(td.value.outerHTML == "<td/>")
    width.value = Some(100)
    assert(td.value.outerHTML == "<td width=100/>")
  }

But implementation such feature in Binding.scala can broke current code with custom tags (if somebody uses suggestion from #4 and has property with type like Option[T])

@Atry, do you think such Binding.scala feature will be useful? /cc @ccamel

[glmars]

Implementation is slightly different from the example in this issue: only Option[String] is supported, you should use option: prefix in an attribute name… please, see test from PR: https://github.com/ThoughtWorksInc/Binding.scala/pull/103/files#diff-1ee69eb8fca397f2ef7f3efd0f631c03

[glmars]

I think I understand you. I am going to create PR (unfortunately, an option to assign this task to myself is disabled on this issue).