Skip CLI's postinstall in case it is not installed globally

[rosen-vladimirov]

Is your feature request related to a problem? Please describe.
When using CLI as a library (i.e. it is declared as a devDependency of my project), every time when I delete node_modules and execute npm install, CLI prompts for enabling autocompletion and some other questions.

Describe the solution you'd like
When CLI is not installed globally, skip all questions.

Describe alternatives you've considered
Having an env flag that will disable these options.

Additional context
To reproduce just call npm install nativescript in any dir (NOTE: do not pass -g flag). CLI will prompt yo for some input.
Postinstall should be executed only when any of the following is executed:

npm i -g nativescript
npm install --global nativescript
yarn global add nativescript
npm i --global nativescript@<version>