Update API and add method for getting a list of unaliased polyfills based on their configured browser ranges

Split out the method for getting a subset of polyfills, from a list of polyfill names (potentially aliased), based on their browser configuration and the supplied UA string
This is the context in which I think we might want to say 'features'.  As in, 'get a list of the polyfills required to support x features in y browser'