Add `rustup component` subcommand

[brson]

The generalized version of rustup target. This command will allow the rust-src package to be installed once rust-lang-deprecated/rust-buildbot#102 lands. One complication here is that the rust-src package has a target equal to "*" in the manifests.

I think the way this might work is rustup component add defaults to the host target, which can resolve to "*". That way you can write rustup component add rust-src.

The command might have the same set of subcommands as rustup target, and e.g. add might look like

rustup.exe-component-add
Add a component to a Rust toolchain

USAGE:
    rustup.exe component add [OPTIONS] <component>

FLAGS:
    -h, --help    Prints help information

OPTIONS:
        --toolchain <toolchain>
        --target <target>

ARGS:
    <target>

[brson]

cc @Diggsey

[Diggsey]

When you say it "resolves to *", do you mean that it will do a reverse match and * will match any target specifier?

Sounds good, although we're getting a lot of subcommands 😛 We might have to start showing a reduced help text with only the key features by default to avoid overwhelming users.