Add --self-contained as its own flag.

I'd like to request `--self-contained` as its own option separate from `-L`.  This is handy when building system packages (rpms and such) where you want to install all the dependencies, regardless of if they're installed, but don't want to use local::lib.  In those cases I set destdir via the `MM_OPT` and `MB_OPT` environment variables.

This is a feature I can't easily implement from outside cpanm.