Add br_spaces parameter to MD009.

doc/Rules.md

@@ -182,9 +182,20 @@ for a description of the problem.
 
 Tags: whitespace
 
+Parameters: br_spaces (number; default: 0)
+
 This rule is triggered on any lines that end with whitespace. To fix this,
 find the line that is triggered and remove any trailing spaces from the end.
 
+The br_spaces parameter allows an exception to this rule for a specific amount
+of trailing spaces used to insert an explicit line break/br element. For
+example, set br_spaces to 2 to allow exactly 2 spaces at the end of a line.
+
+Note: you have to set br_spaces to 2 or higher for this exception to take
+effect - you can't insert a br element with just a single trailing space, so
+if you set br_spaces to 1, the exception will be disabled, just as if it was
+set to the default of 0.
+
 ## MD010 - Hard tabs
 
 Tags: whitespace, hard_tab

lib/rules.js

@@ -237,8 +237,11 @@ module.exports = [
     "desc": "Trailing spaces",
     "tags": [ "whitespace" ],
     "func": function MD009(params, errors) {
+      var brSpaces = params.options.br_spaces || 0;
       params.lines.forEach(function forLine(line, lineIndex) {
-        if (/\s$/.test(line)) {
+        if (/\s$/.test(line) &&
+            ((brSpaces < 2) ||
+             (line.length - line.trimRight().length !== brSpaces))) {
           errors.push(lineIndex + 1);
         }
       });

test/trailing_spaces_br.json

@@ -0,0 +1,6 @@
+{
+  "default": true,
+  "MD009": {
+    "br_spaces": 2
+  }
+}

test/trailing_spaces_br.md

@@ -0,0 +1,4 @@
+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}