Support for template based predicates

[mchlrch]

example CSVW output (propertyUrl):

 {
   "titles":"GRUPPENCODE1",
   "propertyUrl":"https://ld.stadt-zuerich.ch/statistics/property/{GRUPPE1}",
   "valueUrl":"https://ld.stadt-zuerich.ch/statistics/code/{GRUPPENCODE1}"
 },

https://github.com/StatistikStadtZuerich/ld-data/blob/master/input/hdb.csv-metadata.json#L56

In R2RML, I think that would be a rr:predicateMap enclosing a rr:template
https://www.w3.org/TR/r2rml/#predicate-object-map