Use correct output when enabling ANSI colors on Windows (see #39)

tty.go

@@ -15,7 +15,7 @@ func initTerminal(w io.Writer) error {
 		return err
 	}
 
-	enableAnsiColors()
+	enableAnsiColors(w)
 	hideCursor(w)
 	return nil
 }

tty_unix.go

@@ -2,6 +2,8 @@
 
 package tea
 
+import "io"
+
 // enableAnsiColors is only needed for Windows, so for other systems this is
 // a no-op.
-func enableAnsiColors() {}
+func enableAnsiColors(w io.Writer) {}

tty_windows.go

@@ -3,6 +3,7 @@
 package tea
 
 import (
+	"io"
 	"os"
 
 	"golang.org/x/sys/windows"
@@ -10,8 +11,13 @@ import (
 
 // enableAnsiColors enables support for ANSI color sequences in Windows
 // default console. Note that this only works with Windows 10.
-func enableAnsiColors() {
-	stdout := windows.Handle(os.Stdout.Fd())
+func enableAnsiColors(w io.Writer) {
+	f, ok := w.(*os.File)
+	if !ok {
+		return
+	}
+
+	stdout := windows.Handle(f.Fd())
 	var originalMode uint32
 
 	windows.GetConsoleMode(stdout, &originalMode)