Add the ability to run arbitrary commands between waves

### 🚀 Feature Request

Users might want to run commands between waves to do something with the intermediate states of

#### Existing Behavior

No native support exists for this. You could make a bunch of different `typestat.json`s and run them in order, but that's kind of yucky, and won't integrate well with adding exec commands in the config wizard...

#### Change Proposal

Allow an `{ "exec": "..." }` command that can be done between waves.

I'd like to use this in the wizard to ask users if they're using Git and would like commits per operation... probably as a followup: #808