Add a mechanism for testing whether feature is present

Sometimes a particular property/operator is only available on some browsers. The generator will generate the field/methods regardless and the downstream consumer is left to try and feature detect in some other way. The simplest approach may be to just generate a `@JsOverlay boolean supportsMyFeature()` method that will allow caller to detect whether a feature is supported.

It would be too costly to generate `supports*()` methods for every feature so maybe the generator could load the [canisue](https://github.com/Fyrd/caniuse) database and only generate `supports*()` methods if they are not supported compared to a particular base line (IE11?).

See also #29 because these features could be combined and driven by the same data.