Improve LGTM configuration (#2777)

* Testing LGTM

* Update .lgtm.yml

* Update .lgtm.yml

* Update .lgtm.yml

* Update .lgtm.yml

.lgtm.yml

@@ -5,10 +5,11 @@ path_classifiers:
 
 extraction:
   javascript:
+    # https://lgtm.com/help/lgtm/javascript-extraction#customizing-index
     # The `index` step extracts information from the files in the codebase.
     index:
       # Specify a list of files and folders to exclude from extraction.
-      filters:
-        - exclude: plugins/
       exclude:
+        - bower_components/
+        - docs/assets/js/plugins/
         - plugins/