Register a callable to decode rendered content for a given MIME type

Each registered decoder will also define a `#rendered.$MIME` helper
method, where `$MIME` corresponds to the value of the `mime` argument.

=== Arguments

`mime` - Symbol the MIME Type name for the rendered content
`callable` - Callable to decode the String. Accepts the String
                    value as its only argument
`block` - Block serves as the decoder when the
                 `callable` is omitted

By default, ActionView::TestCase defines a decoder for:

* :html - returns an instance of Nokogiri::XML::Node
* :json - returns an instance of ActiveSupport::HashWithIndifferentAccess

Each pre-registered decoder also defines a corresponding helper:

* :html - defines `rendered.html`
* :json - defines `rendered.json`

=== Examples

To parse the rendered content into RSS, register a call to `RSS::Parser.parse`:

```ruby
register_decoder :rss, -> rendered { RSS::Parser.parse(rendered) }

test "renders RSS" do
  article = Article.create!(title: "Hello, world")

  render formats: :rss, partial: article

  assert_equal "Hello, world", rendered.rss.items.last.title
end
```

To parse the rendered content into a Capybara::Simple::Node,
re-register an `:html` decoder with a call to
`Capybara.string`:

```ruby
register_decoder :html, -> rendered { Capybara.string(rendered) }

test "renders HTML" do
  article = Article.create!(title: "Hello, world")

  render partial: article

  rendered.html.assert_css "h1", text: "Hello, world"
end
```