Consider whether the user explicitly specifies a label when applying global custom formatting

Let's say I want to titleize my generated labels.  I can do it:

```ruby
config.label_text = lambda { |label, required| "#{label.titleize}" }
```

But then I can't have a non titled label, even if I specify it.  This:

```ruby
<%= f.input :distributor, label: 'Who provides your cable or satellite service?' %>
```

Will generate this label: ```Who Provides Your Cable Or Satellite Service?```

Yeah, ugly.  It's not what I want, and there's no way to handle it (except maybe with undesirable options like #human_attribute_name or by replacing a major part of the framework.)

This PR is a simple solution that does not break existing code.