detect unstable rustc options and suggest the correct syntax

[tanriol]

There are cases when a user is suggested to use an unstable rustc option (and the suggestion for external-macro-backtrace is even in the rustc output). However, if a user tries to add this option to his cargo command line, the error message just says that this flag is unknown.

It would be a good idea to output something like

note: option `external-macro-backtrace` is a `rustc` option, not a `cargo` one
note: to use it, prefix the `cargo` command line with `RUSTFLAGS="-Z external-macro-backtrace"`

[PoignardAzur]

I'd really like this feature too. I've been burned multiple times by this.