Treatment of DateTime attributes

[pestevez]

We are sending a single property with type date, which is preventing the tool from using two major use cases to parse date-time fields:

• The date is in the form of text e.g. <p>December 15, 2017</p>
• The date is in an attribute with different name e.g. <meta property="article:published_time" content="2018-01-10" />

Should we also support string attributes?