Add markdown support

Markdown support added for episode synopsis and review body.  Test added for episode synopsis markdown support.  Review body markdown support test dependent on review test suite.
You can move the markdown rendering to a helper method that you can then use in any view that you want.

```ruby
# app/helpers/application_helper.rb

module ApplicationHelper
  def render_markdown(markdown)
    extensions = { hard_wrap: true }
    renderer = Redcarpet::Markdown.new(Redcarpet::Render::HTML, extensions)
    renderer.render(markdown)
  end
end
```

Then you can use this helper throughout your views whenever you want to render something as markdown:

```html
<h1><%= @episode.title %></h1>

<%= render_markdown(@episode.synopsis) %>
```

Doing it this way, you don't actually store the HTML in your database, you're storing the markdown. This makes it easier to edit the values in the future, since you're still just editing markdown.