Feature Request: Support !! in bindings

[opus1269]

This makes for a slick way to bind to boolean attributes with non-boolean properties that can be truthy or falsey

For instance, a possible use case from iron-image

       /**
         * Sets a sizing option for the image.  Valid values are `contain` (full
         * aspect ratio of the image is contained within the element and
         * letterboxed) or `cover` (image is cropped in order to fully cover the
         * bounds of the element), or `null` (default: image takes natural size).
         */
        sizing: {
          type: String,
          value: null
        },

...

    <div id="backdrop" hidden$="[[!sizing]]"></div>
    <img id="img" role="none" hidden$="[[!!sizing]]">

Otherwise, you need to put it in a computed binding.

ht to @bicknellr for the idea.

[Jamtis]

Can you please provide a use-case where the hidden tag must be true instead of the sizingvalue.
When I see 'hiding' I think about CSS selectors that apply some displaystyles, however div[hidden] {} would also match <div hidden$="[[sizing]]">.

[TimvdLippe]

This has been previously requested in #2343 . The official response was:

We're not very interested in expanding the vocabulary of expressions right now.
You can always make a computed function that applies a boolean pass over the input:

<my-element attr$="[[bool(numeric_prop)]]"></my-element>

  bool: function(prop) {
    return !!prop;
  }

[opus1269]

@JeremybellEU Thanks for the info.