GenIgnore with more fine-grained language control through "features"

[xkr47]

Having used vertx with the Ceylon programming language for a while, I have noticed that @GenIgnore source has blocked access to interesting features for reasons that don't seem to apply to the Ceylon language.

For example methods returning Map<K,V> or X509Certificate are out of reach while I believe they could be used perfectly well in Ceylon. And #40 lists byte[] as another case.

At the most coarse level one could simply choose to ignore @GenIgnore when generating bindings for specific languages. Alternatively we could perhaps add a (defaulted) parameter to the annotation indicating a feature required from the target language for the declaration to be included. This would give more fine-grained control over which constructs are exposed in which languages. For example the config file for the Ceylon language could indicate that it supports "MAP" and "CERTIFICATE" features (just to give the idea). Perhaps a better-named annotation could be introduced for this and deprecate the original one.