Add an option to skip rust-docs on rust-init (for CI envs)

[gentoo90]

In a CI environments rust-docs component isn't needed but you cant skip its installation during rustup-init.
Add a CLI option --no-docs or --minimal, or even --components with a list of components to install.

[Diggsey]

It might be worth borrowing from cargo's features model, and have something like --no-default-components, and update the manifest to distinguish between required components and installed by default components.

[lilith]

Docs and manpages are huge. Trying to delete them post-install is a moving target. My base docker image is < 300MB, but the Rust layer is 700MB compressed.

[oherrala]

Doc installation seems to be off by default now (since Nov'16):

https://users.rust-lang.org/t/psa-rust-documentation-is-now-optional-and-opt-in-when-installing-via-rustup/8072/22

There also seems to be issue #906 about making the docs installed by default but opt-out if needed. And issue #907 about --minimal flag to install only required components.

[Diggsey]

Closing in favour of above issues

[Rantanen]

Since I just spent a while trying to figure this out and these issues are still dominating Google results, I'll update these with the current status.

Documentation was opt-in in till early 2017, but then they were made required again in March 2017, tracked by #903 in rustup.rs. While that issue wished for "some more options to remove them", that hasn't happened.. yet.

Fortunately #998 seems to have some recent activity towards allowing opt-out for the docs.