add TemplateFunction java interface

needed as a java interface due to clojure not allowing for vararg
parameters in definterface/defprotocol

src/java/clj_jtwig/TemplateFunction.java

@@ -0,0 +1,10 @@
+package clj_jtwig;
+
+import com.lyncode.jtwig.functions.exceptions.FunctionException;
+
+// this is defined in Java only because Clojure defprotocol/definterface don't allow
+// including method definitions with vararg parameters
+
+public interface TemplateFunction {
+	public abstract Object execute (Object... arguments) throws FunctionException;
+}