Add option to convert between SPDX Spec versions #146

The underlying library supports converting between versions if you specify the version when creating the new SPDX document.

Unfortunately, there is no command line option to support this.

There are a couple possibilities:

• Add a new comment like "upgradeSpdxDocument" which will take the new version as a noption
• Add a --version option to the SPDX translator which will use the new version for the translated file

Note that the library only support upgrades, not downgrades.