Introduce expected_output.todo markers

If expected*.todo file is present (alongside of the .css file)
this marks this test as "todo", which can be ignored with
--ignore-todo.

Therefore test which have some output formats passing,
but not the other ones can have a more fine-grained "todo"
granularity. Such tests should not be in the folders
that contain "todo", since then a whole test may be skipped.

Should fix: sass#333

lib/sass_spec/test_case.rb

@@ -12,6 +12,7 @@ def initialize(input_scss, expected_css, error_file, status_file, style, clean,
     # Probe filesystem once and cache the results
     @should_fail = File.file?(@status_path)
     @verify_stderr = File.file?(@error_path)
+    @todo = (@input_path.to_s.include? "todo") || File.file?(@expected_path.sub(/\.css$/, ".todo"))
   end
 
   def name
@@ -51,7 +52,7 @@ def should_fail?
   end
 
   def todo?
-    @input_path.to_s.include? "todo"
+    @todo 
   end
 
   def output