feature: add TAP output option

[andyburke]

Allows for producing TAP-compatible output, which people can run through the various TAP output formatters. Examples:

tap-vibrant

headline

1..7

  group 1

  ✖ 1 should fail
    AssertionError [ERR_ASSERTION]: true == false
        at Object.fn (.../test.js:8:10)
        at Function.self.run (.../node_modules/baretest/baretest.js:56:20)
  ✔ 2 test 2
  ✔ 3 test 3

  group 2

  ✔ 4 test 4
  ✔ 5 test 5

  group 3

  ✔ 6 test 6
  ✔ 7 test 6

tap-spec

  headline

  group 1

      AssertionError [ERR_ASSERTION]: true == false
          at Object.fn (.../test.js:8:10)
          at Function.self.run (.../node_modules/baretest/baretest.js:56:20)
    ✔ test 2
    ✔ test 3

  group 2

    ✔ test 4
    ✔ test 5

  group 3

    ✔ test 6
    ✔ test 7

  Failed Tests: There was 1 failure

    group 1

      ✖ should fail


  total:     7
  passing:   6
  failing:   1
  duration:  6ms