SAN FRANCISCO (Reuters) - Facebook Inc on Thursday launched a catalog feature for its WhatsApp messaging app, it said in a blog post, building out the service's e-commerce tools as it moves slowly toward monetizing the app it bought in 2014 for $19 billion (£14.82 billion).Facebook has been trying to boost revenue from higher-growth units like Instagram and Whatsapp, which has 1.5 billion users. The company's flagship News Feed product continues to draw big ad dollars, but is struggling to..