[Conkeror] New feature: Cookie management interface

[Jeremy Maitin-Shepard]

There is now a simple cookie management interface in modules/cookie.js.

Specifically, it introduces the clear-cookies command, as well as the
user variable cookie_lifetime_policy, which provides an interface for
getting and setting (via session_pref) the Mozilla preferences
network.cookie.lifetimePolicy and network.cookie.lifetime.days.

-- 
Jeremy Maitin-Shepard