Add #name to renderers

Closes #374

From the tests:

```ruby
    assert_equal "markdown", GitHub::Markup.renderer('README.md').name
    assert_equal "redcloth", GitHub::Markup.renderer('README.textile').name
    assert_equal "rdoc", GitHub::Markup.renderer('README.rdoc').name
    assert_equal "org-ruby", GitHub::Markup.renderer('README.org').name
    assert_equal "creole", GitHub::Markup.renderer('README.creole').name
    assert_equal "wikicloth", GitHub::Markup.renderer('README.wiki').name
    assert_equal "asciidoctor", GitHub::Markup.renderer('README.adoc').name
    assert_equal "restructuredtext", GitHub::Markup.renderer('README.rst').name
    assert_equal "pod", GitHub::Markup.renderer('README.pod').name
```

/cc @aroben @github/user-content 
Maybe this should return the name of the gem that will be used for rendering? Presumably the different gems produce different rendering for at least some documents.