Devise an upgrade strategy

[tim-weis]

The Cargo custom command and the corresponding build script it generates are interdependent. With the command allowing to be updated (cargo install cargo-uwp), there needs to be some way to move the build system forward in lock-step.

At this time an update is a manual process: cargo uwp new <path> a new Cargo package, and either move the generated build system artifacts into the existing application, or move the code/templates from the existing application over. Doing either one safely requires #2.

At a minimum it should be possible for the Cargo custom command (and/or the build script) to observe a mismatch in versions. Ideally, though, the upgrade should be fully automatic.