Add todo spec test for libsass issue 108

sass/libsass#108

Author: mgreter

spec/libsass-todo-issues/issue_108/expected_output.css

@@ -0,0 +1,5 @@
+h1 {
+  color: red; }
+
+h2 {
+  color: blue; }

spec/libsass-todo-issues/issue_108/input.scss

@@ -0,0 +1,13 @@
+$a: red;
+
+@mixin f($a: $a) {
+  color: $a;
+}
+
+h1 {
+  @include f;
+}
+
+h2 {
+  @include f(blue);
+}