Require.resolve() in extends cause config not loaded when changed (in vscode)

[unional]

Referencing:
https://github.com/Microsoft/vscode-tslint/issues/45

This is caused by https://github.com/palantir/tslint/blob/master/src/configuration.ts#L139 using require(...) to load the file. require() would cache the file, preventing any change to file to be reloaded.

The proposed change is to load the file directly as before. Since now the path is properly resolve (including using require.resolve()) in resolveConfigurationPath(configFilePath);, we are safe to do that.

cc @egamma

I can create a PR on that tomorrow.