"extends" field is not respected

[beenotung]

When ctsc is running custom tsconfig that extends from other tsconfig, it doesn't follow the extends field.

Below is an example to reproduce the problem.

tsconfig.build.json:

{
  "extends": "./tsconfig.json",
  "exclude": [
    "node_modules",
    "test",
    "dist",
    "**/*spec.ts",
    "scripts",
    "www"
  ]
}

tsconfig.json

{
  "compilerOptions": {
    "module": "commonjs",
    "sourceMap": true,
    "outDir": "./dist",
    "baseUrl": "./"
  },
  "exclude": [
    "node_modules",
    "dist",
    "scripts",
    "www"
  ],
  "include": [
    "src"
  ]
}

When the command ctsc -p tsconfig.build.json is ran, it complaints for No include found in tsconfig /path/to/tsconfig.build.json. This error is unexpected, because the "include" field could be read from the extended file "tsconfig.json" in the example.