Multiple platform apps?

@sixninetynine! Shiv looks rad. I've been lamenting about `pkg_resources` slowing down our Python CLI tools @NerdWallet.

One pex feature we rely on is building a single `.pex` file for multiple platforms (i.e. `pex --platform linux-x86_64 --platform macosx-10.11-x86_64 ...`). We pre-compile all of our own wheel files in a private repo so this means we end up with a single `.pex` full of our custom built wheels (fast builds, happy security folks, etc...)

#7 talks about invoking shiv for a different target, but it doesn't talk about multiple targets.

Is this even possible with Shiv? I hope so, I'd love to contribute back to shiv and explore moving to it!

Start by reading issue #7 and the existing Shiv command-line behavior around selecting a target platform. Compare that behavior with the multi-platform PEX workflow described here, then determine whether a single archive can support multiple targets and document the expected result or implementation scope.

Written by the indexing model from the issue text.