Fixed warning in 64-bit windows build (#24)

* Fixed passing callback arguments as reference

* Another passing as const reference instead of new instance

* Implemented variable number of parameters
-  extended function "function_regex" to pass minimum and maximum parameters. if minimum=maximum, all params are required, if minimum = 0, all params are optional.....
- for single parameter there is different regex (created by the same function_regex function) to allo read sort([1,2,3]) as one parameter
- extended Match to test if group is valid, so only valid params are filled to params vector
- new function arguments_count to test number of arguments passed to callback

* unit tests for updated callbacks with variable number of params

* merge with last version

* Optimized passing arguments as const reference

* Fixed warning on 64-bit windows (conversion from 'size_t' to 'int', possible loss of data)

src/inja.hpp

@@ -347,7 +347,7 @@ struct Parsed {
 	};
 
 	using Arguments = std::vector<ElementExpression>;
-	using CallbackSignature = std::pair<std::string, int>;
+	using CallbackSignature = std::pair<std::string, size_t>;
 };