tsconfig extends support

[Peter-Juhasz]

TypeScript: 2.1.4
Version: 3.1.3

Expected behavior:
Applies inheritance and interpolates the tsconfig file references in the extends attribute.

Running tsc works as expected.

Actual behavior:
createProject does not support tsconfig inheritance, it passes only the tsconfig which references another one without interpolating the base config file.

Even if I change extends to a file which does not even exists, there is no error, just different compiler options than set in the base tsconfig.

gulpfile.js

var tsBuildProject = typescript.createProject("tsconfig.build.json");
gulp.task("app:scripts:ts:build", function () {
    return tsBuildProject.src()
        .pipe(sourcemaps.init())
        .pipe(tsBuildProject()).js
        .pipe(ngAnnotate())
        .pipe(concat(paths.concatAppJsDest))
        .pipe(uglify())
        .pipe(sourcemaps.write("."))
        .pipe(gulp.dest("."));
});

tsconfig.json

{
    "compileOnSave": false,
    "compilerOptions": {
        "target": "ES5"
    }
    ...
}

tsconfig.build.json

{
    "extends": "./tsconfig",
    ...
}

[ivogabe]

Duplicate of #459