Allow the user to customise the columns displayed in `py list`

Is your feature request related to a problem? Please describe.
I don't find the default display of py list particularly useful. The aliases take up a lot of space and aren't very helpful (they are easily deducible from the rest of the data). Ideally, I'd like to be able to choose more useful information to display in place of that column.

Describe the solution you'd like
The --format option allows you to display a single data item (--format=exe). That's fine, but it would be nice to be able to choose multiple values. Something like --format=tag,exe.

Describe alternatives you've considered
You can get this sort of report by piping --format=json into jq, or into a custom formatter, but that's fairly clumsy.

Additional context
Alternative to #86. The approach here is probably overkill, but I've filed it as an option in case changing the default format is unacceptable.

Start by locating the py list command's --format handling and compare its single-item output with the existing JSON output. Define how comma-separated values such as tag,exe should be parsed and rendered, then verify that multiple selected fields appear in the requested order without breaking the existing single-value behavior.