Allow overriding of default configured form_class

This is especially useful with boostrap forms where you might want to
switch to a `form-inline` layout even though the default is a
`form-horizontal`.

e.g.

```ruby
config.form_class = "form-horizontal" # configured default
```

```ruby
simple_form_for @user, form_class: "form-inline"
```

I recognize that this was already proposed and closed in #657, but I
feel like this new implementation is more straight forward, cleaner, and
more discoverable without passing around non-standard html attributes.

Just throwing this out there.