fix: prompter for mail on postinstall is very obligatory

[rosen-vladimirov]

On postinstall CLI checks if the user had subscribed for NativeScript newsletter if not, prompts for email. However, the messages seems obligatory, while it is not.
Reformat it, so the users can easily skip it in case they do not want to subscribe for the newsletter.

PR Checklist

• The PR title follows our guidelines: https://github.com/NativeScript/NativeScript/blob/master/CONTRIBUTING.md#commit-messages.
• There is an issue for the bug/feature this PR is for. To avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it.
• You have signed the CLA.
• All existing tests are passing: https://github.com/NativeScript/nativescript-cli/blob/master/CONTRIBUTING.md#contribute-to-the-code-base
• Tests for the changes are included.

What is the current behavior?

The prompter on postinstall seems mandatory.

What is the new behavior?

The prompter on postinstall does not seem so mandatory.

Fixes/Implements/Closes #[Issue Number].

[rosen-vladimirov]

run ci