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.

---

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.
Because of the stupid 79 character limit, you end up writing like this:
~~~~
@commands.arg('command',
              help='something '
              'that can be '
              'very long '
              'very very '
              'long')
~~~~