fix(postinstall): allow usage of NPM config for ghAuthToken

[petermuessig]

Using the GH Octokit API to often (unauthenticated) may lead to rate limit or blocking of requests. Therefore, the generator supports setting the NPM config:

npm config set easy-ui5_ghAuthToken ghp_xxxx

With this configuration you won't reach any rate limit or blocking anymore and setting it in your NPM config should be safe enough.

Fixes #100

[petermuessig]

This fix is FYI - I'll tested it locally and this fixes the problems of some colleagues with the postinstall when the API rate limit has been reached.

[petermuessig]

I'll go ahead and release 3.4.0