Please consider making the session cookie name configurable

Or alternatively allow it to be scoped to a path within the domain.

My problem is that I'm running two gitit instances on the same server (one public and another private). I have found that I can't stay logged into both at the same time, because they both use the cookie named `sid` to store the session id. I took notes on my attempt to fix it [here](http://chrismaloney.org/notes/Gitit#two-gitit-instances-cant-coexist-on-the-same-server).

Here was my initial attempt to fix it: https://github.com/Klortho/gitit/commit/b7974c01484056dcb57f33db6c0aee7a60218198

But I abandoned the effort when it started to look like the `sid` string is hardcoded in a library used by your program, maybe here: http://hackage.haskell.org/package/salvia-sessions-1.0.0/docs/src/Network-Salvia-Handler-Session.html. Not knowing any Haskell, I am out of my depth.

I have a work-around, and that is just to use a different domain for each of the wikis, but that's not ideal.

Thanks for such a great program.