adds Program.Write method

[tgirod]

This method gives access to Program.output so one can write directly to the terminal's output.

Use case

Your app is not fullscreen, and you want to print out some stuff to the terminal while the application is running.

You can do so by using fmt.Println, but looking at bubbleatea's code, there is a mutex for accessing Program.output. So this method politely grabs the mutex before trying to write to Program.output.

Considerations

Maybe we don't want tea.Program to implement the Writer interface - in which case the method can be renamed to something else.

[meowgorithm]

Closing in favor of #179, as mentioned in #267.

[meowgorithm]

Just a note that this functionality is now available in master and will be available in the next release. For details see #249.