Add zvm megascript (feedback needed)

NOTE: This is a work in progress. I want to get some feedback on the general approach here before I spend too much time cleaning it up.

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Addresses https://github.com/zerovm/zerovm-cli/issues/59.

This adds a `zvm` command line executable which simply glues `zpm` and
`zvsh` together into a single script, with a unified set of commands.

`zpm` subcommands are imported verbatim and can be run in the manner of
`zvm new`, `zvm bundle`, etc. `zvsh` command args have been placed
underneath the `zvm run` subcommand, but have otherwise not been
changed.
I see you're just following the pattern you laid down earlier, but what does this do? (I haven't seen other Python projects that "provide" anything like this.)