Option for variable expansion when setting environment variables

This PR adds an `expand` option to `magic_shell_environment` to switch the quotes from single to double, so variables within `value` can be expanded. An example of where this might be useful is setting `$PATH`, previously a `value` of "/my/custom/path:$PATH" would not be expanded.

I made this an option rather than simply replacing the quotation directly as some existing users may depend on the lack of expansion.