make multiple calls to start() harmless

src/index.ts

@@ -66,11 +66,15 @@ export function Interpreter(machine:Machine_T, initialContext:any, initialStateN
   return interpreter;
 }
 export function start(interpreter:Interpreter_T){
+  if(interpreter.isPaused === true){
     interpreter.isPaused = false;
     processEvents(interpreter);
+  }
 }
 export function pause(interpreter:Interpreter_T){
+  if(interpreter.isPaused === false){
     interpreter.isPaused = true;
+  }
 }
 
 /**  Helper function for `send()`