Add values md5 hash to section cache key

@astashov please take a look

It allows do the following:

```erb
<% cache(DeadSimpleCMS.sections.site_announcement) do %>
  ...
<% end %>
```

or 

```ruby
module Cells
  module Bar
    class Cell < App::Cell
      cache(:main)
        [
          DeadSimpleCMS.sections.site_announcement.cache_key,
          Treatment.for("Experiment")
        ].join("_")
      end
  end
end
```