rtk errors when commands have `--`

[fnando]

$ cargo test -- --nocapture
    Finished `test` profile [unoptimized + debuginfo] target(s) in 0.30s
    …

$ rtk cargo test -- --nocapture

error: unexpected argument '--nocapture' found
  tip: a similar argument exists: '--features'
  tip: to pass '--nocapture' as a value, use '-- --nocapture'
Usage: cargo test --features <FEATURES> [TESTNAME] [-- [ARGS]...]
For more information, try '--help'.

This is related to #286, which doesn't actually pass through unrecognized commands, like stated in the generated instructions file.