Allow trailing spaces in indented code blocks (#191)

test/code_block_fenced.md

@@ -9,6 +9,12 @@ And here is more text
 This is a code block that won't trigger.
 ```
 
+```text
+This is code
+ 
+with a line with trailing spaces. {MD009:14}
+```
+
 But we'll do another:
 
     And this {MD046}

test/code_block_indented.md

@@ -14,4 +14,8 @@ But we'll do another:
     And this
     will.
 
-Final text is here
+One last one:
+
+    This is code
+    
+    with an empty indented line.

test/trailing_spaces_br.md

@@ -2,3 +2,5 @@ This line has a single trailing space {MD009}
 This line has two trailing spaces and should be allowed  
 This line has three trailing spaces {MD009}   
 This line has four trailing spaces {MD009}    
+ 
+{MD009:5}