define includes option via command line & includes.yml

Author: bweigel

tests/base/includes.yml

@@ -0,0 +1,6 @@
+default:
+  - handler.py
+foobar: 
+  - handler.py
+  - foobar
+empty: null

tests/base/serverless.yml

@@ -27,13 +27,12 @@ custom:
     individually: false
     useStaticCache: false
     useDownloadCache: false
-    include: [handler.py]
 
 package:
   individually: ${opt:individually, self:custom.defaults.individually}
   exclude:
     - '**/*'
-  include: ${opt:include, self:custom.defaults.include}
+  include: $file(./includes.yml):${opt:include, 'default'}
 
 functions:
   hello: