Add support for cjs custom mapping files (#178)

Author: perrin4869

lib/migration-strategy/custom.js

@@ -12,7 +12,7 @@ function loadCustomizations(serverless, plugin) {
 
   try {
     // support exporting a custom stack map
-    return /\.js$/.test(customPath)
+    return /\.c?js$/.test(customPath)
       ? require(customPath)
       : serverless.utils.readFileSync(customPath);
   } catch (e) {