Make property operators extensible

[vincent-zurczak]

#34 has introduced a new assertion type on property values.
We now have equality (« to be ») and regular expressions (« to match »). We could imagine other ones too.

If new ones had to appear, it would be nice to refactor the base regexp and not increase their number.
Said differently, we should think about an extensible grammar rather than copying the base regexp for the DETIK syntax.