ng config doesn't parse array brackets

[Meithal]

🐞 Bug report

Command (mark with an x)

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

Is this a regression?

Can't tell

Description

Ng config tells that you can provide a json path, but array subscription doesn't seem to work.

https://angular.io/cli/config

🔬 Minimal Reproduction

Create a new project, angular.json file should have an asset entry with 2 entries, to access the table you can do
ng config "projects.<your-project>.architect.build.options.assets" and it will output an array
But if you do
ng config "projects.<your-project>.architect.build.options.assets[0]" it will tell "Value cannot be found", expected is "src/favicon.ico" or whatever is here...

🔥 Exception or Error

Value cannot be found.

🌍 Your Environment

Angular CLI: 7.3.9
Node: 8.9.4
OS: win32 x64
Angular: 7.2.15
... animations, common, compiler, compiler-cli, core, elements
... forms, http, language-service, platform-browser
... platform-browser-dynamic, router, service-worker

Package                           Version
-----------------------------------------------------------
@angular-devkit/architect         0.13.9
@angular-devkit/build-angular     0.13.9
@angular-devkit/build-optimizer   0.13.9
@angular-devkit/build-webpack     0.13.9
@angular-devkit/core              7.2.4
@angular-devkit/schematics        7.3.9
@angular/cdk                      7.3.7
@angular/cli                      7.3.9
@angular/flex-layout              7.0.0-beta.24
@angular/material                 7.3.7
@angular/pwa                      0.12.4
@ngtools/webpack                  7.3.9
@schematics/angular               7.3.9
@schematics/update                0.13.9
rxjs                              6.5.2
typescript                        3.2.4
webpack                           4.29.0

Anything else relevant?

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