[WIP] Add Listing of Installed Components

This feature adds the ability to only list installed components
rather than all available components.

Author: naftulikay

src/rustup-cli/rustup_mode.rs

@@ -291,6 +291,7 @@ pub fn cli() -> App<'static, 'static> {
                 .subcommand(
                     SubCommand::with_name("list")
                         .about("List installed and available components")
+                        .arg(Arg::with_name("installed").long("installed").help("List only installed toolchains"))
                         .arg(
                             Arg::with_name("toolchain")
                                 .help(TOOLCHAIN_ARG_HELP)