Add docs command

This PR adds "rustup docs" command which is aliased to "rustup doc".
Closes #928.

Author: topecongiro

src/rustup-cli/rustup_mode.rs

@@ -304,6 +304,7 @@ pub fn cli() -> App<'static, 'static> {
             .arg(Arg::with_name("command")
                 .required(true)))
         .subcommand(SubCommand::with_name("doc")
+            .alias("docs")
             .about("Open the documentation for the current toolchain")
             .after_help(DOC_HELP)
             .arg(Arg::with_name("book")