Support for shell variable expansion in files

**Describe the feature you'd like**
Following #174 we could expand shell variables inside the files like

```
byexample '$HOME/foo.py'
```

This should work even if the file is in an argument file (those you load from command line with `@args`).

Shell variable expansion should happen before globbing and file expansion (#174 )

**Additional context (optional)**
Package to expand variables: [expandvars](https://github.com/sayanarijit/expandvars)

**Note:** not really sure if this is a feature that we want. I don't have a concrete example, in contrast to #174 .