This commit inlines https://github.com/tj/go-termd for a few reasons:
- off the shelf it relies on a broken, erroneously released version of
blackfriday (a markdown parser)
- based on discussion with ampinsk, there are some tweaks we'd like to
make to markdown rendering beyond what the library exposes now
- it's a small library (around 300 sloc)
This commit only:
- messes with go.mod to fix the blackfriday issues
- adds an inclusion note
- renames the package
