sort: add "dictionary-order" flag.

The flag makes 'sort' command ignore non-dictionary symbols
(non-alphanumeric and non-spaces). The only difference with GNU sort is
that it takes ALL alphanumeric symbols, not only ASCII ones.

tests/fixtures/sort/dictionary_order.expected

@@ -0,0 +1,3 @@
+bbb
+./bbc
+bbd

tests/fixtures/sort/dictionary_order.txt

@@ -0,0 +1,3 @@
+./bbc
+bbd
+bbb

tests/test_sort.rs

@@ -65,6 +65,11 @@ fn test_ignore_case() {
     test_helper("ignore_case", "-f");
 }
 
+#[test]
+fn test_dictionary_order() {
+    test_helper("dictionary_order", "-d");
+}
+
 #[test]
 fn test_multiple_files() {
     new_ucmd!()