This is a proof-of-concept to enable passing a function for thedomain config option to dynamically modify the cookie domain based on the incoming request.

It does so by adding a call to a newmodifyOptsForRequest helper anywherecookies.set is called.

modifyOptsForRequest creates a copy ofopts, checks whether theopts.domain value is a function, and replaces thedomain value with the result of that function if so.

An example of how this might be used:

app.use(session({  domain: (ctx, opts) => {    return ctx.get("origin") || DEFAULT_ORIGIN;  }});

This specifically addresses#188, but it should be easy to extend this PR to support the other cookie config options (likehttpOnly,maxAge, etc.). If this approach looks good and maintainers would be interested in the fix, I'd be happy to update this PR to properly do so - let me know!