stopwatch: auto_start option, make pause cumulative

doc/docs.md

@@ -1494,7 +1494,7 @@ You can also use stopwatches to measure just portions of your code explicitly:
 ```v
 import time
 fn main(){
-    sw := time.new_stopwatch()
+    sw := time.new_stopwatch({})
     println('Hello world')
     println('Greeting the world took: ${sw.elapsed().nanoseconds()}ns')
 }