feature: add groups and allow toggling quiet output

[andyburke]

Allow for test groups and more verbose output. Submitted with the expectation of discussion on these changes, since they're significant, even if they don't break backwards compatibility.

Examples:

No groups, quiet output (current behavior, but output is slightly changed)

headline
• • • • • • 
✓ 6

Groups + default quiet output

headline

  group 1 • • 
  group 2 • • 
  group 3 • • 
✓ 6

Groups + quiet output disabled

headline
  group 1
    first test name ✓
    second test name ✓
  group 2
    first test name ✓
    second test name ✓
  group 3
    first test name ✓
    second test name ✓

✓ 6