Add ability to specify multiple paths via --code-coverage=@<path>

[davidanthoff]

For the VS Code extension it would be useful if we could specify multiple paths here, so that if a file is in any of those folders it would be included. So support something like --code-coverage=@/foo/bar,/bar/foo with the ability for an arbitrary number of folders.

[davidanthoff]

@MilesCranmer do you think this is something you would know how to tackle? Or maybe @vtjnash?