feat(cli): Add color to clap help/errors

[epage]

This adds styling to clap help and errors to make it easier to scan. This uses Cargo's styling (via clap-cargo) for consistency across the project.

• For consistency, my main priorities are end-user commands like rustup and cargo and not plumbing commands like rustfmt and rustc.
• We've not yet decided how to handle this through an official crate, so doing it unofficially for now.

To ensure consistency, the Discussion sections were updated.

• I was limited in my use of color within Discussion to match at least the initial approach we've taken in Cargo. I worry that if we color every literal, placeholder, etc then it will be a sea of colors and lose the benefit. Instead color is limited to items we want to draw extra attention to or visually big items to help set them apart
• Cargo doesn't have sub-headers, so I had to define that myself for rustup completions
• I could have used something like color-print to do the styling at compile time but that would require duplicating Cargo's styling which would have been harder to maintain. Instead, we style at runtime so we can use clap-cargo.

[djc]

Looks great, thanks!

[rami3l]

We might indeed want to address https://rust-lang.zulipchat.com/#narrow/channel/490103-t-rustup/topic/More.20rustup.2Fcargo.2Frustc.20consistency/near/545410015 in addition.

Otherwise this looks pretty nice, thanks!

[epage]

Everything should now be styled.

[djc]

Nit: sorry, one more that I forgot in the last round: please put this at the bottom.

[epage]

Went ahead and did this but this is one of the rare cases where it feels better at the top to me.

In general, the way I view organizing a source file is it should act like a tree with the top item(s) at the top and branch out from there, so the top item(s) act as a ToC and provide context. In that case, constants are noise. However, in this case, this has all of the styling elements together and now we're splitting them up.

[djc]

I don't find that a very strong argument. Lots of modules where some items are imported while other items are defined near the bottom.