feat(@angular/cli): allow comments in .angular-cli.json

[thorn0]

Also allow trailing commas and unquoted object keys.

Closes #5610

[hansl]

We already have our own Json parser that supports those (basically Json5 support, and has AST with positioning): https://github.com/angular/devkit/tree/master/packages/angular_devkit/core/src/json

[thorn0]

@hansl Okay, I used it.
Unfortunately, it doesn't support trailing commas, but I guess I'll open a separate issue or a PR for that. Comments are a bigger issue anyways.

[thorn0]

Updated devkit to 0.4.0 to get support for trailing commas.

[thorn0]

Okay, I'm not sure how to update devkit properly. Seems like it's not that simple and shouldn't be part of this PR. So I'm leaving this PR as is.

[thorn0]

Rebased it onto master. @hansl Please have a look.

[clydin]

Closing as 6.0 has a new configuration system which brings support for comments.

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