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.
Deprecate cookie expiration config without params

Moving forwards, the lambda used to set the cookie expiration should
take a single parameter that will be populated with all available
cookies that have been set.