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:

```ruby
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:

```ruby
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.