Added optional printer argument to spinner action

[calebstewart]

This PR adds an optional argument to the printer action which provides access to the tea.Printf and tea.Println functionality. I added the Printer interface with two methods: Printf and Println. I also added a accessiblePrinter type which just proxies the calls to fmt.Printf and fmt.Println.

When called normally (without accessibility), the tea.Program is passed in as the Printer object. When called with Accessible(true), then a dummy accessiblePrinter object is passed in.

The Spinner interface is left backwards compatible. The Action() method still takes a func(), but a new method is introduced called ActionWithPrinter() which takes a func(Printer) function instead.

[maaslalani]

Ohhh, this is a very nice idea and good implementation @calebstewart! Thank you so much.

Let me just think about what we should call this to make it more easily discoverable. I almost wonder if we could have people simply print to a buffer on the spinner to achieve this effect i.e. with fmt.Sprintln(spinner.Out, "This is a test") 🤔

[caarlos0]

maybe passing a io.Writer is simpler 🤔