feat: record groups

[AndersDJohnson]

Fixes #252.

Also includes opt-in support to write fixtures matching specific group URL patterns to a top-level group sub-directory (at fixtures/posts/suite-name instead of fixtures/suite-name), e.g.:

{
  "groups": {
    "users": "/api/v(.*)/users",
    "posts": {
      pattern: "/api/v(.*)/posts",
      directory: "posts" // or 'true' to use group name
    }
  }
}

Will document in a future PR.

[coveralls]

Pull Request Test Coverage Report for Build 1229

• 28 of 28 (100.0%) changed or added relevant lines in 4 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.02%) to 94.39%

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Totals
Change from base Build 1228:	0.02%
Covered Lines:	807
Relevant Lines:	832

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💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 1232

• 29 of 29 (100.0%) changed or added relevant lines in 4 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.04%) to 94.413%

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Totals
Change from base Build 1228:	0.04%
Covered Lines:	808
Relevant Lines:	833

---

💛 - Coveralls