Support caching

[johnsonj]

Syntax

View:

<% progressive_render cache_keys: [@val] do %>
    <%=@val %>
<% end %>

Expected Behavior

On first load, the partial is rendered as a placeholder and filled in. On subsequent loads it is in the cache and is part of the initial page load.

Open Issues

• Adapt to specific cache stores? Rails.cache ok?
• What additional cache params do we need?