feat(@angular/cli): add eject command

[filipesilva]

Blocked by angular/devkit#946 and angular/devkit#947

[ghost]

Is eject coming back soon?

[alexeagle]

We'll follow up with @IgorMinar about the options for delivering this

[ngbot]

Hi @filipesilva! This PR has merge conflicts due to recent upstream merges.
Please help to unblock it by resolving these conflicts. Thanks!

[alexeagle]

@IgorMinar we discussed again, we don't plan to add back the eject command, instead we will follow the proposal to provide some docs on our wiki and a webpack config or seed repo that shows how to do it.

@filipesilva can you locate the proposal for that?

@StephenFluin as FYI, and to veto if needed

[alexeagle]

ngx-build-plus handles many use cases that would require ng eject.
We are currently not planning to add this command back.

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