Add separate config option for cookie maxage

[artagel]

Related to #91, Some users may want to have a separate maxage for the vouch cookie compared to the JWT age.
This is useful if you want the cookie to be deleted when a browser is closed, while retaining a logical JWT maxage.

[artagel]

I can help add this.

[bnfinet]

Sounds like a reasonable request.

Is there ever a situation where vouch.cookie.maxAge would be longer than vouch.jwt.maxAge? That condition should probably be at least log.Warn but maybe log.Fatal.

Thanks much @artagel

[artagel]

Yeah, I think you are right. makes no sense to make a cookie that can live longer than the JWT.