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.
The normal style for Python programs is to let the indentation do the grouping:

```
xxxxxxxxxx:
    xxxxxxxxx:
        xxxx
        xxxxxxx
            xxxxxx
        xxxx

    xxxxxxxxxxxx:
        xxxx
```

Your style lets the parenthesis do the grouping. This how you delimit things in C, Java and other curly-brace languages. So I would write the calls like:

```python
@commands.arg('command',
              help='something that can be '
              'very long')