Support RDiscount 2.

Fixes #2747.

One error:

```text
  1) Failure:
TestRdiscount#test_: rdiscount should render toc.  [/Users/parker/jekyll/jekyll/test/test_rdiscount.rb:19]:
<"<h1 id=\"Header+1\">Header 1</h1>\n\n<h2 id=\"Header+2\">Header 2</h2>\n\n<p>\n <ul>\n <li><a href=\"#Header+1\">Header 1</a>\n  <ul>\n  <li><a href=\"#Header+2\">Header 2</a>  </li>\n  </ul>\n </li>\n </ul>\n\n</p>"> expected but was
<"<a name=\"Header.1\"></a>\n<h1>Header 1</h1>\n\n<a name=\"Header.2\"></a>\n<h2>Header 2</h2>\n\n<p><ul>\n <li><a href=\"#Header.1\">Header 1</a>\n <ul>\n  <li><a href=\"#Header.2\">Header 2</a></li>\n </ul>\n </li>\n</ul>\n\n</p>">.
```