Add useStrict option to ALLOWED_KEYS.

src/rollup.js

@@ -24,7 +24,8 @@ const ALLOWED_KEYS = [
 	'onwarn',
 	'outro',
 	'plugins',
-	'sourceMap'
+	'sourceMap',
+	'useStrict'
 ];
 
 export function rollup ( options ) {

test/test.js

@@ -76,7 +76,7 @@ describe( 'rollup', function () {
 			return rollup.rollup({ entry: 'x', plUgins: [] }).then( function () {
 				throw new Error( 'Missing expected error' );
 			}, function ( err ) {
-				assert.equal( 'Unexpected key \'plUgins\' found, expected one of: banner, dest, entry, exports, external, footer, format, globals, indent, intro, moduleId, moduleName, onwarn, outro, plugins, sourceMap', err.message );
+				assert.equal( 'Unexpected key \'plUgins\' found, expected one of: banner, dest, entry, exports, external, footer, format, globals, indent, intro, moduleId, moduleName, onwarn, outro, plugins, sourceMap, useStrict', err.message );
 			});
 		});
 	});