Relax restrictions on unknown feature directives

Instead of forcibly always aborting compilation, allow usage of
 #[warn(unknown_features)] and related lint attributes to selectively abort
 compilation. By default, this lint is deny.
auto merge of #10680 : alexcrichton/rust/relax-feature-gate, r=thestinger

Instead of forcibly always aborting compilation, allow usage of
 #[warn(unknown_features)] and related lint attributes to selectively abort
 compilation. By default, this lint is deny.