generate more than 1 Server-Timing header

[DanielRuf]

It would be good to have it configurable, if one (big) or multiple (short) headers should be created.

https://github.com/tuupola/server-timing-middleware/blob/master/src/ServerTimingMiddleware.php#L133

[DanielRuf]

See also #25 (comment)

[tuupola]

Yeah, agreed. Quickly thinking having one flag for choosing between single header and multiple headers would be a clean solution. Having a configurable header size in bytes feels more error prone.