Contextual voice commands are not yet supported by the GDK. There is a feature request that you can follow here: https://code.google.com/p/google-glass-api/issues/detail?id=273
However, the necessary features are already available in some libraries and you can use them as long as the GDK doesn't support this natively. I've described the necessary steps to extract and use the library in this answer: Glass voice command nearest match from given list