add Cirru syntax highlighting

demo/kitchen-sink/docs/cirru.cirru

@@ -0,0 +1,36 @@
+-- https://github.com/Cirru/cirru-gopher/blob/master/code/scope.cr,
+
+set a (int 2)
+
+print (self)
+
+set c (child)
+
+under c
+  under parent
+    print a
+
+print $ get c a
+
+set c x (int 3)
+print $ get c x
+
+set just-print $ code
+  print a
+
+print just-print
+
+eval (self) just-print
+eval just-print
+
+print (string "string with space")
+print (string "escapes \n \"\\")
+
+brackets ((((()))))
+
+"eval" $ string "eval"
+
+print (add $ (int 1) (int 2))
+
+print $ unwrap $
+  map (a $ int 1) (b $ int 2)