Allow to set alternative cookie name

[amedveshchek]

I really like this simple and yet powerful package, which I use in my project.

What I found impossible to do is:

• to overcome the limit of collisions with other packages which already use their own session cookie;
• also it would be nice to have a way to extend the 4096 bytes limit, by simply creating an additional cookie with another name.
• from architectural perspective, it's also can be useful to logically separate cookies of different types: for session, for flash messages, for bookmarks etc.

In that relation I'm offering to add the following change.

[donchev7]

Would be great to have this.