Implement custom variable definitions

-) Implement custom definitions (Variable or Definition as keyword) of the form:
    Variable|Definition=VARNAME; [Value=]SOMECOMPLEXVALUE
-) Such defined variables can be used in all further rules.
-) Add "Condition=" keyword to indicate that a certain value is a condition (e.g. a variable defined as implemented here).
   This is needed to distinguish single-variable (or function-call) conditions from shipping costs, where the "Shipping=" was left out.
   Previously, only terms that contained a comparison operator were detected to be conditions. This excluded conditions like
   "issubset(...)" or "MyZIPConditions" (i.e. functions returning boolean values and boolean values stored in variables).
-) The rule-matching code was moved to its own function, so the logic to distinguish variable definitions and (later on) modifiers
   is in only one place.