Support --target-dir

[pbor]

I quickly tried to use it as a drop in replacement for cargo test in our build scripts, but we pass --target-dir to cargo and this does not seem to be supported:

error: unexpected argument '--target-dir' found

  tip: a similar argument exists: '--re'

[sourcefrog]

At least as a workaround you should be able to use --cargo-arg --target-dir=D, or -C for short.)