Implement HTML reports

[greglook]

After test results are available, we can generate an HTML report to provide a more human-friendly format for consuming the results. This doesn't have to be fancy, but should be substantially easier to parse than the raw EDN result data.

• Show an aggregate of number of tests, total elapsed time, aggregate assertions, etc.
• Detailed info for each test (bonus: expand/collapse for more info).
• Bonus: some kind of test-case timeline showing when things ran. (more useful for concurrent test runs)