Markdown filter support for Haml 4

in response to https://github.com/middleman/middleman/issues/662#issuecomment-15880076

This is just the failing tests, seems like the app context isn't being passed to hamls markdown filter:

relevant output from the tests:

```bash
Scenario: Markdown filter in Haml uses our link_to and image_tag helpers # middleman-more/features/markdown_redcarpet_in_haml.feature:25
    Given a fixture app "markdown-in-haml-app"                             # middleman-core/lib/middleman-core/step_definitions/builder_steps.rb:19
    And a file named "config.rb" with:                                     # aruba-0.5.1/lib/aruba/cucumber.rb:19
      """
      set :markdown_engine, :redcarpet
      activate :directory_indexes
      """
    And a file named "source/link_and_image.html.haml" with:               # aruba-0.5.1/lib/aruba/cucumber.rb:19
      """
      :markdown
        [A link](/link_target.html)

        ![image](blank.gif)
      """
    Given the Server is running at "markdown-in-haml-app"                  # middleman-core/lib/middleman-core/step_definitions/server_steps.rb:60
    When I go to "/link_and_image/"                                        # middleman-core/lib/middleman-core/step_definitions/server_steps.rb:65
      undefined method `link_to' for #<Object:0x007fdebab39318> (NoMethodError)
      ./middleman-core/lib/middleman-core/renderers/redcarpet.rb:62:in `link'
      ./middleman-core/lib/middleman-core/renderers/redcarpet.rb:49:in `evaluate'
      (eval):5:in `render_with_options'
      ./middleman-core/lib/middleman-core/core_extensions/rendering.rb:260:in `block in render_individual_file'
      ./middleman-core/lib/middleman-core/util.rb:157:in `fetch'
      ./middleman-core/lib/middleman-core/core_extensions/rendering.rb:259:in `render_individual_file'
      ./middleman-core/lib/middleman-core/core_extensions/rendering.rb:148:in `render_template'
      ./middleman-core/lib/middleman-core/sitemap/resource.rb:128:in `block in render'
      ./middleman-core/lib/middleman-core/util.rb:60:in `instrument'
      ./middleman-core/lib/middleman-core/application.rb:222:in `instrument'
      ./middleman-core/lib/middleman-core/sitemap/resource.rb:14:in `instrument'
      ./middleman-core/lib/middleman-core/sitemap/resource.rb:114:in `render'
      ./middleman-core/lib/middleman-core/core_extensions/request.rb:255:in `process_request'
      ./middleman-core/lib/middleman-core/core_extensions/request.rb:204:in `block in call!'
      ./middleman-core/lib/middleman-core/core_extensions/request.rb:203:in `catch'
      ./middleman-core/lib/middleman-core/core_extensions/request.rb:203:in `call!'
      ./middleman-core/lib/middleman-core/core_extensions/request.rb:188:in `call'
      ./middleman-core/lib/middleman-core/step_definitions/server_steps.rb:66:in `/^I go to "([^\"]*)"$/'
      middleman-more/features/markdown_redcarpet_in_haml.feature:40:in `When I go to "/link_and_image/"'
    Then I should see "/link_target/"                                      # middleman-core/lib/middleman-core/step_definitions/server_steps.rb:73
    Then I should see 'src="/images/blank.gif"'                            # middleman-core/lib/middleman-core/step_definitions/server_steps.rb:77
```

Let me know if y'all need anything, I'm happy to work on getting it fixed -- I just don't know the internals well enough to know where to start.