Add support for defining 2 levels deep in blocks (ie. `ava.default`)

[levibuzolic]

  rules: {
    'no-only-tests/no-only-tests': ['error', {
      block: ['ava.default'],
      focus: ['only']
    }]
  }

Will now raise lint errors for code that looks like:

ava.default.only(() => {})

Closes #14