Edition should be part of the "short" subset of rustc opts

[peterjoel]

The default edition is 2015, but the option to change this in the "extended" list, so it is only discoverable with rustc --help -v, rather than just rustc --help.

Users should be encouraged to use 2018 where possible, but the omission of the edition from the default help list makes it less likely that it will be found for users not using Cargo.