option to download expressions from SPDX.org's github license list

[glen-nicol]

As mentioned by the nuget docs the licenses.nuget.org URLs are meant for viewing in a browser. It would be very helpful to be able to get the raw license template instead of a full HTML page so that I can incorporate these into a larger aggregate of all the 3rd party dependencies of an application.

Would a PR with options to pull from the spdx.org github be welcomed to do this? They have the raw templates in txt form as well as JSON data files of all the license options in a single JSON array file.

[sensslen]

@glen-nicol pull requests are always welcomed.