support extends field in tsconfig

[beenotung]

Resolved issue #4

Example

cli:

ctsc -p tsconfig.build.json

tsconfig.json:

{
  "compilerOptions": {
    "module": "commonjs",
    "moduleResolution": "node",
    "target": "es2015",
    "outDir": "dist"
  },
  "include": [
    "src/**/*.ts",
    "test/**/*.ts"
  ],
  "exclude": [
    "node_modules"
  ]
}

tsconfig.build.json:

{
  "extends": "./tsconfig.json",
  "include": [
    "src/**/*.ts"
  ],
  "exclude": [
    "node_modules"
  ]
}

Before this PR, it will throw TypeError: Cannot read property 'outDir' of undefined