Add stylus-mode auto installation

core/prelude-packages.el

@@ -162,6 +162,7 @@ PACKAGE is installed only if not already present.  The file is opened in MODE."
     ("\\.scala\\'" scala-mode2 scala-mode)
     ("\\.scss\\'" scss-mode scss-mode)
     ("\\.slim\\'" slim-mode slim-mode)
+    ("\\.styl\\'" stylus-mode stylus-mode)
     ("\\.swift\\'" swift-mode swift-mode)
     ("\\.textile\\'" textile-mode textile-mode)
     ("\\.thrift\\'" thrift thrift-mode)