Added a setting for the max combined header size value

I've used the fork of @PowerKiKi from FirePHP to test the `X-Wf-Max-Combined-Size` header features but Chrome still reports the `net::ERR_RESPONSE_HEADERS_TOO_BIG` error.
I'm not sure if it's still a bug in @PowerKiKi fork but after setting a max allowed size of i.e. 255kB (instead of 256kB) it works.
So I've created a small setting using excactly this default value.

@PowerKiKi fork may be found here: [headers_too_big branch](https://github.com/PowerKiKi/firephp-core/tree/headers_too_big)