Handle showSource config

Made changes according to code review feedback.

Author: hy9be

src/plots/plots.js

@@ -257,7 +257,7 @@ plots.previousPromises = function(gd) {
  * Add source links to your graph inside the 'showSources' config argument.
  */
 plots.addLinks = function(gd) {
-    if(gd._context.showLink === false) return;
+    if(!gd._context.showLink || !gd._context.showSources) return;
 
     var fullLayout = gd._fullLayout;