add configuration options for the session cookie

[cdepillabout]

When a user logins in, Goat Guardian creates a session cookie called _GG_SESSION.

The settings for the cookie are defined here:

goat-guardian/src/GoatGuardian.hs

Lines 324 to 331 in cc51574

	let setCookie =
	defaultSetCookie
	{ setCookieName = "_GG_SESSION"
	, setCookieValue = encryptedUserKey
	, setCookiePath = Just "/"
	, setCookieMaxAge = Just $ 60 * 60 * 24 * 3650 -- 10 years
	, setCookieHttpOnly = True
	}

It would be nice to allow the operator the ability to change these settings.