Add support for attributedString

[KingOfBrian]

This will probably be one of the more complicated builders. I'd prefer we generate BonMot code.

<attributedString key="attributedText">
     <fragment content="I agree that I have read and understand the ">
        <attributes>
             <color key="NSColor" red="0.58823529411764708" green="0.58823529411764708" blue="0.58823529411764708" alpha="1" colorSpace="calibratedRGB"/>
             <font key="NSFont" size="12" name="HelveticaNeue"/>
             <paragraphStyle key="NSParagraphStyle" alignment="left" lineBreakMode="wordWrapping" baseWritingDirection="natural"/>
         </attributes>
     </fragment>
     <fragment content="terms &amp; conditions">
         <attributes>
             <color key="NSColor" red="0.14117647058823529" green="0.59607843137254901" blue="0.54509803921568623" alpha="1" colorSpace="calibratedRGB"/>
             <font key="NSFont" size="12" name="HelveticaNeue-Bold"/>
             <paragraphStyle key="NSParagraphStyle" alignment="left" lineBreakMode="wordWrapping" baseWritingDirection="natural"/>
         </attributes>
     </fragment>
 </attributedString>