Per field layout option

This feature adds a `:layout` option to any field. If you have a large horizontal form, but want a few inline fields mixed in, they can be added to override the layout of the form. Basic usage:

```erb
<%= bootstrap_form_for(@user, layout: :horizontal) do |f| %>
  <%= f.email_field :email %>
  <%= f.text_field :feet, layout: :inline %>
  <%= f.text_field :inches, layout: :inline %>
  <%= f.form_group do %>
    <%= f.submit %>
  <% end %>
<% end %>
```