Improving flag --save-dev for add package in devDependecies

[kyusupov33]

Bug Report or Feature Request (mark with an x)

- [ ] bug report -> please search issues before submitting
- [x] feature request

Command (mark with an x)

- [ ] new
- [ ] build
- [ ] serve
- [ ] test
- [ ] e2e
- [ ] generate
- [x] add
- [ ] update
- [ ] lint
- [ ] xi18n
- [ ] run
- [ ] config
- [ ] help
- [ ] version
- [ ] doc

In our @ngxs/schematics package we need to add it in devDependencies, but by default in angular-cli flag --save-dev will be true. You can make sure on it in this file: https://github.com/angular/angular-cli/blob/master/packages/angular/cli/tasks/npm-install.ts.

We can send PR and add ability to add package with ng add command in devDependencies.

[eranshmil]

https://github.com/angular/angular-cli/blob/master/packages/angular/cli/commands/add-impl.ts#L46
I think the save default should be false, because of this comment.
But I guess that's a breaking change.

[eranshmil]

@hansl implemented the autosave in 729d407.
Also, this is a duplicate of #12003.

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