Currently, users are required to install and manage their own Rust toolchains. Let's make the user's life easier, and do it for them, along the lines of the multirust script for Unix. Feature list:

• Select one of multiple toolchains for use (corresponding to Rust releases/release channels)
• Able to download and install/update toolchains
• Per-project toolchains
• Notifications to update with the ability to auto-update
• Support for both mingw and msvc targets

/cc @brson