Support vim's ability to use number and/or relativenumber

Apparently not everyone likes to have the current line number shown when using relativenumber, so vim now offers the choice. To handle this for hopefully everyone, this adds g:only_toggle_relativenumber

When enabled (the default), only relativenumber is toggled (thus obviously assuming number is on); Else number will also be toggled as needed, to never have both options at the same time.