Ability to display errors as a summary instead of inline

Currently, we display errors as a help block for a particular field. Add the ability to turn this off so we can display them as a summary instead.

I was thinking that passing an option to the builder might work. It could default to 'inline', but also accept 'summary'?

Something like: 

```ruby
bootstrap_form_for @user, errors: :inline
```

or 

```ruby
bootstrap_form_for @user, errors: :summary
```