PROV-997 Added skipIfExpression setting to exporter

It uses the ExpressionParser that was written for the importer (see
http://docs.collectiveaccess.org/wiki/Expressions). You can reference
the value of the current mapping as variable ^value. Next we’re going
to implement a system that allows you to define your own variables in
exporter mappings which can then be referenced in these expressions. At
the moment they’re pretty useless.
PROV-998 Added variables to exporter

This allows you to use all the available exporter features to assign
dynamic values to an identifier and then later reference it as variable
in expressions for skipIfExpression. This is the only use so far but
potentially we could also allow using those as source or in templates
as well. The notion of saving something and then using it in a
different context later can be very useful in data exports.