Multi-page markdown report

A multi-page markdown report of a test session can be generated after the session has completed:

$ canary run ./basic
...

$ canary report markdown create
INFO: Markdown report written to TestResults/canary-report.md

$ cat TestResults/canary-report.md
# Canary Summary

| Site | Project | Not Run | Timeout | Fail | Diff | Pass | Invalid | Cancelled | Total |
| --- | --- | --- | --- | --- | --- | --- | --- | --- | --- |
| build-32997738-project-1150422-canary-wm | None | 0 | 0 | 0 | 0 | 0 | 0 | 0 | [2](MARKDOWN/Total.md) |