Allow content_fors in cached blocks

[brenes]

When a block is cached if the view has a content_for that content_for won't be executed once the block is cached.

We may allow some configuration that says "this view must be rendered in this content for". Something like

NoCms::Blocks.block_layouts = {
    'block' => {
      template: 'my_block',
      fields: {
        something: :string,
        another: :string
      },
      content_for: {
         ready_js: 'my_block_ready_js',
         sidebar: 'my_block_sidebar'
      }
  }
}

And the helper would render those views around a content_for.

This is just a first idea, maybe there's a better system.