Enable simpler implementation of `remember me`

The lambda for `Clearance.configuration.cookie_expiration` can now
optionally take an argument. If that argument is present, `session` will
call the configured lamdba passing the entire collection of cookies
available in session. This allows implementation of remember_me without
monkey-patching clearance.

Anyone wanting to implement remember_me would now:

* add the remember_me checkbox to the login form.
* override sessions_controller create to set the remember_me cookie, like so:

```
def create
  set_remember_me
  super
end

private

def set_remember_me
  if params[:remember_me]
    cookies.permanent[:remember_me] = true
  else
    cookies.delete(:remember_me)
  end
end
```

* Configure clearance using something like this:

```
config.cookie_expiration = ->(cookies) do
  if cookies['remember_me']?
    1.year.from_now
  else
    nil
  end
end
```

The last two steps could conceivable be extracted to their own gem to
make this simpler, if desired.