Added the 'csvw' prefix to the RDFa initial context

In accordance with the way the RDFa initial context is defined, the list of predefined prefixes has been extended by adding 'csvw'. The corresponding vocabulary URI is http://www.w3.org/ns/csvw#, and it stands for the metadata vocabulary for tabular data, like CSV or TSV, which was published as a Recommendation at the very end of 2015.

Reminder: the RDFa initial context provides a number of prefixes and a number of prefix-less properties that RDFa processors should recognize with the user being required to add those via a @prefix declaration.