added option to form_for that disables automatic creation of labels

Title is kinda self-explanatory, default behaviour hasn't changed.

Here's an example of how it works:
```erb
<%= form_for @article, auto_labels: false do |f| %>
  <%= f.text_field :title %> # no label
  <%= f.datetime_select :publish_date, label: 'Will be published on' %> # labelled 'Will be published on'
  <%= f.check_box :important, label: true %> # labelled 'Important'
<% end %>
```