Add configuration to add class globally to inputs.

Thinking about this feature, after implement it, I don't know if is a good idea allow the user to set a class globally. This can make more difficult to change the default class on the inputs.

Something like this:

```
f.input :name, :input_class => false, :input_html => { :class => "test" }
```

Or if we have to do this, we need a better implementation.