Inconsistent path resolution

[dantleech]

Some paths are relative to the config, while others to the CWD:

  "$schema": "vendor/phpbench/phpbench/phpbench.schema.json",
  "runner.path": "../../tests/Benchmark",
  "runner.bootstrap": "phpbench-bootstrap.php",
  "runner.file_pattern": "*Bench.php",
  "storage.xml_storage_path": ".tools/phpbench/data/storage",
  "report.outputs": {
    "html": {
      "renderer": "html",
      "title": "Benchmarks",
      "path": ".tools/phpbench/data/html/index.html"
    }
  }

Suggest to fix this in 2.0 and make them all relative to the config file in which they are defined.