Add test for data attr prefixing #79 (#81)

Author: RadValentin

test/fixtures/transform.css

@@ -23,3 +23,7 @@ body {
 .class-b {
   color: deepskyblue;
 }
+
+[data-world] .d {
+  padding: 10px 20px;
+}

test/fixtures/transform.expected.css

@@ -23,3 +23,7 @@ body.hello {
 .hello .class-b {
   color: deepskyblue;
 }
+
+.hello [data-world] .d {
+  padding: 10px 20px;
+}