Avoid ES6 syntax (#350)

Allows the plugin to work in IE11 without adding babel, etc.

Author: benmccann

src/plugin.js

@@ -84,7 +84,7 @@ function directionEnabled(mode, dir, chart) {
 	} else if (typeof mode === 'string') {
 		return mode.indexOf(dir) !== -1;
 	} else if (typeof mode === 'function') {
-		return mode({chart}).indexOf(dir) !== -1;
+		return mode({chart: chart}).indexOf(dir) !== -1;
 	}
 
 	return false;