Implement rustup CLI

[brson]

This adds the rustup command as described in #129.

Major changes:

• A bare rustup updates all tracked channels and self-updates.
• All the --copy-local, --link local and --installer options
  are gone. There is just rustup channel link, which covers the
  most common reason for creating custom toolchains.
• add-target, list-targets, and remove-targets are all under
  the target subcommand, and all operate on the current toolchain
  instead of needing to specify it explicitly.
• Override comands are all under the override subcommand.
• Lesser commands for modifying toolchains are under the toolchain
  subcommand, but I decided multirust update and multirust update nightly were useful enough to stay its own subcommand.
• The UI for updating all channels is changed from multirust-rs
  slightly: both the update status and channel revision are displayed
  in the same place.
• I've removed green from 'info' logging and used it only for
  successful updates to make it more impactful.

Note there are several ways to update now: rustup updates
everything, rustup update updates the current toolchain, and rustup update [toolchain] updates a specific toolchain.

--help text is still missing.

Next steps are:

• get this live and get some internal feedback
• change multirust-setup to rustup-setup
• add deprecation warnings to multirust
• move metadata dir to ~/.rustup
• rewrite readme and rustup.rs
• set up rustup.rs, win.rustup.rs and sh.rustup.rs

[alexcrichton]

Oh man I'm excited!

Can you remind me what the self update workflow looks like? If this is merged and published, and then I multirust self update, will I get rustup as a command as well linked into place?

[brson]

@alexcrichton Yes, when this lands then after the next self update everybody will have a rustup command.

[alexcrichton]

🍍

r=me

[brson]

There's a lot more to do here and parts of the CLI are going to change, but I'm merging this to get the forward progress.