TMI: Finishing command-line arguments

I just submitted a pull request that implements most of the commonly-used rustc flags as flags for rustpkg. This took longer than it would have taken if I had taken the approach of just passing any "unknown" flag to rustc, but I think it's useful for rustpkg to do a bit of checking so as to tell you if you're passing nonsensical combinations of flag and command. For example, most rustc flags don't make sense unless you're either building or installing a package, so rustpkg errors out in those cases (for example, if you do rustpkg list --opt-level=3).

I didn't add a test case for every flag, and there's still some refactoring of boilerplate that could be done, but I prioritized getting this done on schedule over completeness.