fix(horizontal rule): revert backwards incompatibility change

Horizontal rule syntax allows up to 3 spaces preceding dashes or
asterisks. Commit da8fb53 wrongfully removed that. This commit
puts that back.

Closes #317

dist/showdown.js

@@ -1258,9 +1258,9 @@ showdown.subParser('blockGamut', function (text, options, globals) {
 
   // Do Horizontal Rules:
   var key = showdown.subParser('hashBlock')('<hr />', options, globals);
-  text = text.replace(/^( ?-){3,}[ \t]*$/gm, key);
-  text = text.replace(/^( ?\*){3,}[ \t]*$/gm, key);
-  text = text.replace(/^( ?_){3,}[ \t]*$/gm, key);
+  text = text.replace(/^ {0,2}( ?-){3,}[ \t]*$/gm, key);
+  text = text.replace(/^ {0,2}( ?\*){3,}[ \t]*$/gm, key);
+  text = text.replace(/^ {0,2}( ?_){3,}[ \t]*$/gm, key);
 
   text = showdown.subParser('lists')(text, options, globals);
   text = showdown.subParser('codeBlocks')(text, options, globals);

src/subParsers/blockGamut.js

@@ -14,9 +14,9 @@ showdown.subParser('blockGamut', function (text, options, globals) {
 
   // Do Horizontal Rules:
   var key = showdown.subParser('hashBlock')('<hr />', options, globals);
-  text = text.replace(/^( ?-){3,}[ \t]*$/gm, key);
-  text = text.replace(/^( ?\*){3,}[ \t]*$/gm, key);
-  text = text.replace(/^( ?_){3,}[ \t]*$/gm, key);
+  text = text.replace(/^ {0,2}( ?-){3,}[ \t]*$/gm, key);
+  text = text.replace(/^ {0,2}( ?\*){3,}[ \t]*$/gm, key);
+  text = text.replace(/^ {0,2}( ?_){3,}[ \t]*$/gm, key);
 
   text = showdown.subParser('lists')(text, options, globals);
   text = showdown.subParser('codeBlocks')(text, options, globals);

test/issues/#317.spaces-before-hr.html

@@ -0,0 +1,2 @@
+<hr />
+<hr />

test/issues/#317.spaces-before-hr.md

@@ -0,0 +1,3 @@
+   ---
+
+   - - -