Strip ^N and ^O from preview output

https://github.com/junegunn/fzf/issues/391#issuecomment-257090266

e.g. fzf --preview 'printf "$(tput setaf 2)foo$(tput sgr0)bar\nbar\n"'

src/curses/curses.go

@@ -682,7 +682,13 @@ func (w *Window) Erase() {
 }
 
 func (w *Window) Fill(str string) bool {
-	return C.waddstr(w.win, C.CString(str)) == C.OK
+	return C.waddstr(w.win, C.CString(strings.Map(func(r rune) rune {
+		// Remove ^N and ^O (set and unset altcharset)
+		if r == 14 || r == 15 {
+			return -1
+		}
+		return r
+	}, str))) == C.OK
 }
 
 func (w *Window) CFill(str string, fg int, bg int, a Attr) bool {