Support for shell variable expansion in files

[eldipa]

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

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