Add upgrade code option to the initialize and print subcommands

[volks73]

An -U,--upgrade-code option that takes a string should be added to the cargo wix init and cargo wix print subcommands. This would override the automatically generated GUID for the product's upgrade code with an existing GUID generated via some other mechanism or tool.

This would be useful if porting an installer build from another tool to use the cargo wix subcommand. It would also be useful to create "ephemeral" installers in CI workflows (See #88).

[volks73]

Implemented as of 39fe04e.

I decided to name the option "upgrade-guid" instead of "upgrade-code" to be eventually consistent with the "path-guid" option. I am not sure if the -U short name will stay very long. It was available, but the -P that would be used for the "path-guid" option is already used and I am not sure if this option will be common enough to warrant a short name.