Render markdown embedded in schema

[effigies]

When we take field descriptions or issue messages from the schema, there may be markdown, or at the very least line breaks and tabs that cause the rendering to be uglier than it might be.

For the web validator, markdown rendering should be relatively straightforward. For the CLI validator, I've found https://github.com/vweevers/markdown-to-ansi to help us convert to ANSI.

One feature of the schema is that there are sometimes links that start with SPEC_ROOT/. This should be replaced with https://bids-specification.readthedocs.io/en/<bids-version>/.

[effigies]

Another option: https://github.com/trabus/markdown-it-terminal