Discover and verify samples embedded in Markdown

[remark](https://github.com/remarkjs/remark) seems like it may be capable of extracting what we need from Markdown documents and executing samples.

Given that markdown is more widely used than Asciidoctor, this may greatly increase the possible user base of this library.

Start by evaluating the linked remark project for Markdown sample extraction and execution capabilities. Define how embedded samples should be discovered and verified, and confirm that completion means Markdown documents can be processed with the same sample-verification goal described for this library.

Written by the indexing model from the issue text.