feat(application): add spec option

[s5s5]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/nestjs/nest/blob/master/CONTRIBUTING.md
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Other... Please describe:

What is the current behavior?

Issue Number: nestjs/nest-cli#2327

What is the new behavior?

The command nest g app some-app --no-spec will not add test files.

Does this PR introduce a breaking change?

• Yes
• No

Other information

I've used unit tests to make sure the logic is correct, but I'm trying to use an npm link to get the local @nestjs/cli to call my modified @nestjs/schematics without success (when debugging @nestjs/cli, I find that he prints out logs for bin/nest.js, and logs for the other files don't show up).
Maybe I've succeeded, but I'm not sure. 🤣

[kamilmysliwiec]

lgtm