rxvt-unicode-plugins.perls: 2.2 -> 2.3

This upgrade brings a new option to the keyboard-select plugin:

  URxvt.keyboard-select.clipboard: If true, copy to clipboard too

and removes the now deprecated clipboard and url-select plugins. The
reasons for deprecating these two plugins are described at:
https://github.com/muennich/urxvt-perls/blob/master/deprecated/README.md