Handle licenses not found in the SPDX license list with the "Other licensing information" section

[alilleybrinker]

The SPDX standard includes a lengthy list of known licenses. In many cases, projects just use those licenses, and no further license specification is necessary. However, sometimes projects don't use those licenses, or claim to use them but include their own modifications. In these cases, SPDX defines a mechanism for specifying non-standard licenses.

cargo-spdx needs to support the following:

• Detecting when non-standard licenses are in use.
• Producing the proper outputs in the SPDX document to represent those licenses.