MRG: make libs positional arguments in roosh script

```
> roosh -h
usage: roosh [-h] [-l] [-u] filename [libs [libs ...]]

positional arguments:
  filename
  libs          libraries required to read contents of the ROOT file

optional arguments:
  -h, --help    show this help message and exit
  -l            don't print the intro message
  -u, --update  open the file in UPDATE mode (default: READ)
```