Add test case for backslash escapes

tests/backslash_escapes.in.md

@@ -0,0 +1,9 @@
+\*not emphasized*  
+\<br/> not a tag  
+\[not a link](/foo)  
+\`not code`  
+\* not a list  
+\# not a heading  
+\[foo]: /url "not a reference"  
+\&ouml; not a character entity  
+1\. not a list  

tests/backslash_escapes.out.md

@@ -0,0 +1,9 @@
+\*not emphasized\*  
+\<br/> not a tag  
+\[not a link\](/foo)  
+\`not code\`  
+\* not a list  
+\# not a heading  
+\[foo\]: /url "not a reference"  
+\&ouml; not a character entity  
+1\. not a list  

tests/it.rs

@@ -150,3 +150,10 @@ fn similar_heading_different_casing() {
 fn tables_with_html() {
     assert_toc!("tables_with_html");
 }
+
+#[test]
+fn backslash_escapes() {
+    // Regression test #21
+    // Backslash-escaped elements should still be escaped.
+    assert_toc!("backslash_escapes");
+}