Double negation support

[nazar-pc]

It is sometimes convenient to use double negation.
For instance, sometimes you have numeric property and you want other element to have active property if only number is non-zero, in this case double negation is useful in order to pass boolean value instead of numeric:

<my-element attr$="[[!!numeric_prop]]"></my-element>

It is much nicer to have just attr instead of attr="5".
Probably, related to #2182, but I'm not sure.

[dfreedm]

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;
  }