SPDX License Identifiers

FORD currently has its own metadata field for specifying the license on a file. However, it should also be able to parse the standard [`SPDX-License-Identifier`](https://spdx.dev/ids/) and provide a link to the appropriate license if found. It looks like this should be reasonably simple with [nexB/license-expression](https://github.com/nexB/license-expression) for parsing. I can submit a PR if there is interest.