Add multi life downloading example
user
```ruby
# encoding: utf-8

require 'tty-spinner'
require 'pastel'

pastel = Pastel.new
spinners = TTY::Spinner::Multi.new("[:spinner] Downloading files...")

['file1', 'file2', 'file3'].each do |file|
  spinners.register("[:spinner] #{file}") do |sp|
    sleep(rand * 5)
    sp.success(pastel.green("success"))
  end
end

spinners.auto_spin
```
assistant