Improve contexts regexp in data/jokes

- The old "[^ \t\n\r]*" behaves exactly like 'Prelude.words'
- The new "\\w*" uses the extended "word" characters class which
  excludes punctuation etc.

XML Schema Character Classes:
http://books.xmlschemata.org/relaxng/relax-CHP-9-SECT-4.html#relax-CHP-9-SECT-4.3