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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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.