Generates a HTML for nested form

Examples of use:

• Standard

  <%= formex_nested f, :user_info %>

• Set a form template for nested form

  <div class="form-horizontal">
    <%= formex_nested f, :user_info, template: Formex.Template.BootstrapHorizontal %>
  </div>

• Use your render function

  <%= formex_nested f, :user_info, fn subform -> %>
    <%= formex_row subform, :section %>
    <%= formex_row subform, :organisation_cell %>
  <% end %>

• Template and render function

  <div class="form-horizontal">
    <%= formex_nested f, :user_info, [template: Formex.Template.BootstrapHorizontal],
      fn subform -> %>
      <%= formex_row subform, :section %>
      <%= formex_row subform, :organisation_cell %>
    <% end %>
  </div>