Need ability to set fields to clearButtonMode

``` ruby
rmq.app.alert(
title: "Text Field",
message: "My style is :input",
style: :input,
placeholder: "whatever",
clearButtonMode: true,
default_text: "My Default Text"
) do |action_type, fields|
puts "you entered '#{fields[:text].text}"
end
```

The standard clear button is displayed at the right side of the text field, when the text field has contents, as a way for the user to remove text quickly. This button appears automatically based on the value set for this property.

The default value for this property is UITextFieldViewModeNever.