Re: [heycam/webidl] Need pattern for feature detecting dictionary members (#107)

Could you clarify a bit what would be the semantics of `<class>.supports()`? Does it just mean "it's on my IDL", i.e. no actual support is necessary/guaranteed? Do you think it would be confusing to do:
```
const myOptions = {...};
if (whatever.support(myOptions)) {
  let a = whatever(myOptions); 
}
```
and then `a` fails to be created? Or worse, if `myOptions` are actually ignored when creating it?

And if the meaning of it is "actually supported", it means it should never be ignored? (If we are looking at this as a feature detection). Otherwise, we wouldn't be really addressing it and `supports` would be more of a necessary but sufficient condition? And if so, then I'm not sure I get the utility of this outside of another unreliable feature detection scheme.

My feeling is that we seem to be trying to solve an issue of required/optional parameters, which the "have an options dictionary argument" created. 

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