Just another example of missing string interpolation

[Andrea Fontana]

Real life example. Render a string like this:

```
\x1b[2K\r\x1b[1mProgress:\x1b[0m 50% \x1b[1m\tSpeed:\x1b[0m 15.5 
KB/s
```

now (error prone and difficult to debug, try to code it!):


```
stderr.write(format("%s\r%sProgress:%s %5.1f%% %s\tSpeed:%s %6.1f 
%s%s", clear, white, clear, progress, white, clear, curSpeed, 
unit));
```

(or with string concat, good luck!)

vs:

```
stderr.write("${clear}\r{$white}Progress:${clear}${progress}% 
\t${white}Speed:${clear} ${curSpeed} ${unit}");
```

Andrea