CLI option to convert a single SPDX Expression (String) to RDF

[hoijui]

While this is trivial for a single license without expression, it would be nice to have this for more complex expressions. This would be useful for a wide variety of tools that take user-input, and generate RDF, where licensing is part of the input.

(a few years ago, I opened a similar issue somewhere else, which you @goneall responded to)