Don't terminate references with other references

ref #54

lib/parser_block.js

@@ -15,7 +15,7 @@ var _rules = [
   [ 'blockquote', require('./rules_block/blockquote'), [ 'paragraph', 'reference', 'blockquote', 'list' ] ],
   [ 'hr',         require('./rules_block/hr'),         [ 'paragraph', 'reference', 'blockquote', 'list' ] ],
   [ 'list',       require('./rules_block/list'),       [ 'paragraph', 'reference', 'blockquote' ] ],
-  [ 'reference',  require('./rules_block/reference'),  [ 'reference' ] ],
+  [ 'reference',  require('./rules_block/reference'),  [ ] ],
   [ 'heading',    require('./rules_block/heading'),    [ 'paragraph', 'reference', 'blockquote' ] ],
   [ 'lheading',   require('./rules_block/lheading') ],
   [ 'html_block', require('./rules_block/html_block'), [ 'paragraph', 'reference', 'blockquote' ] ],