Include aria-required attribute on required fields.

[Closes #780]

lib/simple_form/inputs/base.rb

@@ -66,6 +66,7 @@ module SimpleForm
 
         @input_html_classes = @html_classes.dup
         @input_html_options = html_options_for(:input, input_html_classes).tap do |o|
+          o['aria-required'] = true if has_required?
           o[:readonly]  = true if has_readonly?
           o[:disabled]  = true if has_disabled?
           o[:autofocus] = true if has_autofocus?