allow .pug to be threated as jade (#3019)

see https://github.com/pugjs/pug/issues/2184 for more details

lib/linguist/languages.yml

@@ -1705,6 +1705,7 @@ Jade:
   type: markup
   extensions:
   - .jade
+  - .pug
   tm_scope: text.jade
   ace_mode: jade
 

samples/Jade/hello.pug

@@ -0,0 +1,9 @@
+doctype html
+html
+  head
+    meta(charset='utf-8')
+    link(rel='stylesheet', type='text/css', href='main.css')
+    title Hello Pug
+  body
+    #text
+      include page