Expose LastOutput to enable scrolling output

This allows printing output above the spinner. In our use-case,
we have logs streaming above the spinner using:

`fmt.Fprintf(wtr, "\r%s\n")`

By exposing LastOutput, we can immediately print the spinner again.
Without this field, flashing occurs due to the spinner delay.

spinner.go

@@ -180,7 +180,7 @@ type Spinner struct {
 	Prefix     string                        // Prefix is the text preppended to the indicator
 	Suffix     string                        // Suffix is the text appended to the indicator
 	FinalMSG   string                        // string displayed after Stop() is called
-	lastOutput string                        // last character(set) written
+	LastOutput string                        // last character(set) written
 	color      func(a ...interface{}) string // default color is white
 	Writer     io.Writer                     // to make testing better, exported so users have access. Use `WithWriter` to update after initialization.
 	active     bool                          // active holds the state of the spinner
@@ -315,7 +315,7 @@ func (s *Spinner) Start() {
 					}
 					outPlain := fmt.Sprintf("\r%s%s%s", s.Prefix, s.chars[i], s.Suffix)
 					fmt.Fprint(s.Writer, outColor)
-					s.lastOutput = outPlain
+					s.LastOutput = outPlain
 					delay := s.Delay
 
 					if s.PostUpdate != nil {
@@ -403,11 +403,11 @@ func (s *Spinner) UpdateCharSet(cs []string) {
 // erase deletes written characters on the current line.
 // Caller must already hold s.lock.
 func (s *Spinner) erase() {
-	n := utf8.RuneCountInString(s.lastOutput)
+	n := utf8.RuneCountInString(s.LastOutput)
 	if runtime.GOOS == "windows" && !isWindowsTerminalOnWindows {
 		clearString := "\r" + strings.Repeat(" ", n) + "\r"
 		fmt.Fprint(s.Writer, clearString)
-		s.lastOutput = ""
+		s.LastOutput = ""
 		return
 	}
 
@@ -418,7 +418,7 @@ func (s *Spinner) erase() {
 	// of the line. If n is 2, clear entire line. Cursor position does not
 	// change.
 	fmt.Fprintf(s.Writer, "\r\033[K")
-	s.lastOutput = ""
+	s.LastOutput = ""
 }
 
 // Lock allows for manual control to lock the spinner.