supress rewriting of progress bar if redirecting to file

fixes #17 

```
$ bin/python  demo-app.py --foo think | tee /dev/null
[                    ]   1% 1/200
[====>               ]  26% 51/200 (5s; 14s left)
[=========>          ]  51% 101/200 (10s; 9s left)
[==============>     ]  76% 151/200 (15s; 4s left)
[===================>] 100% 199/200 (19s)
$
```