feat(cli): Adding support for custom uglify options in .angular-cli.json

[jgodi]

Description

Adding an option into the .angular-cli.json to provide custom options to the UglifyJsPlugin that is used for production builds.

Why?

We are using some older libraries inside our Angular application that when we build for production we require the compress: { keep_fnames: true } on the UglifyJsPlugin plugin. The CLI does not currently support this, so I added a way to pass any options to the UglifyJsPlugin.

The .angular-cli.json can now have the following property:

"apps": [{
    "uglifyOptions": {
        "mangle": {
            "keep_fnames": true
        }
    }
}]

Then this will just merge the already defaulted options that were there with any new options that the user needs or wants.

This issue was brought up and closed in a few PRs and tickets, but there was never a solution. (#1662 & #3167)

[Brocco]

Thank you for putting in the time for this PR. However the CLI's current approach to build customization is via the eject option (ng eject) which will give users full control over the build process.

There are plans in the future to allow incremental changes to the existing build process without having to eject but that level of granularity is not ready yet.

[nschipperbrainsmith]

@Brocco is this still the way to go about it by ejecting the full config in case I wish to modify the mangle keep_fnames option within the models/webpack-configs/production.js ? I would love to be able to simply change that one option to say don't mangle my names.

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