Add the ability to configure whether components are installed

[alexcrichton]

Two use cases:

• It'd be nice to have the ability to either opt-out of rust-docs installation or opt-in to tracking it. I don't have much of a preference either way, but I personally at least don't use the component much so it'd be nice to download/copy less.
• For MinGW toolchains I have MSYS installed and get weird errors sometimes when the rust-mingw component is installed because C code is sometimes compiled with my MSYS toolchain and then linked with rust-mingw's toolchain (causing errors).

[brson]

For rust-docs at least, we're just going to blanket convert it to an optional component, so it won't be installed by default.

Maybe rust-mingw should also be converted to an optional component until we figure out something better to do with it.

[brson]

Adding an ability to optionally install required components seems wrong.

[brson]

This issue can be for a general rustup component subcommand, which we'll need before rust-docs can become optional.

[alexcrichton]

I'm fine leaving rust-mingw installed-by-default for now, I was just thinking that there's some way to opt out of installing it (which is persisted)

[brson]

Maybe this speaks to a need for a new type of component, one that is not required but is installed by default. Though i'm still more inclined to relegate rust-mingw to the ndk layer.

[brson]

(Or maybe we just don't have to be so strict about what "required" means and just allow people to mix and match components in ways that break).

[Diggsey]

Superceded by #905