Add support for markdown output (useful for GitHub Actions summaries)

[mfridman]

GitHub recently added the ability to "supercharge" GitHub Actions with job summaries.

https://github.blog/2022-05-09-supercharging-github-actions-with-job-summaries/

Let's add an output flag to allow modifying the output from the default basic output, to the markdown equivalent.

I added some crude code to see what that could look like, and I quite like it. There is some potential here to add some Go-specific summary with text information.

---

Things that could be improved.

• Add ability to sort the summary table, either by package name or by elapsed time (ASC and DESC)
• In a separate section of the markdown consider adding collapsable sections to expose the individual tests for that package, sortable by test name or by elapsed time

Example:

+---------+---------+------------------------------------------------+---------+
| STATUS  | ELAPSED |                      TEST                      | PACKAGE |
+---------+---------+------------------------------------------------+---------+
| 🟢 PASS |    0.57 | TestCompareStrings                             | strings |
| 🟢 PASS |    0.12 | TestCaseConsistency                            | strings |
| 🟢 PASS |    0.00 | TestBuilder                                    | strings |
| 🟢 PASS |    0.00 | TestBuilderGrow                                | strings |
| 🟢 PASS |    0.00 | TestBuilderWrite2                              | strings |
| 🟢 PASS |    0.00 | TestBuilderWrite2/Write                        | strings |
| 🟢 PASS |    0.00 | TestBuilderWrite2/WriteRune                    | strings |
| 🟢 PASS |    0.00 | TestBuilderWrite2/WriteRuneWide                | strings |
...