Give all of the plugins long names

Plugins in general should have long, descriptive names in Cargo.toml; this is already laid out in the book. 

For the example plugins, maybe just prefixing with "cimvr_example_".
This helps avoid name collisions.

TODO: Append the semver major number to the `pkg_namespace!()` macro? Can be retrieved via macros from the stdlib (`env!()`).