feat: Add diff mode

[maratori]

This PR adds new --mode diff and closes #111

Example:

--- goldens/separator.in
+++ goldens/separator.in
@@ -1,17 +1,17 @@
 Comma separator:
 static class Foo {
   // keep-sorted-test start
-  "Foo",
+  "Bar",
-  "Bar",
+  "Baz",
-  "Baz"
+  "Foo"
   // keep-sorted-test end
 }
 
 Comma separator with comments:
-// keep-sorted-test start sticky_comments=yes
-"Foo",
+// keep-sorted-test start sticky_comments=yes
 // Sticky comment
 "Bar",
-"Baz"
+"Baz",
+"Foo"
 // Trailing comments
 // keep-sorted-test end