Progress bar produces long outputs

[amallia]

Related to #417

The progress bar causes a problem if run a long process as it produces one line per second.

The usual solution to that is to detect whether we're speaking to the TTY or a file. Here's an example for POSIX systems (we'd have to replace stdin with stdout):

https://stackoverflow.com/a/7601564

We should define somewhere a is_tty function that will implement this for all platforms, and use that in our code.