Seed data command

I'm not sure if seeding should go into this lib, or if it should be an external lib. But being able to write seed data as JSON or JS could be pretty handy. Just imagine...

**cats.json**
```json
{
"resource": "cats",
"data": [
{ },
{ },
{ }
]
}
```

and then...

`pls seed ./seeds`

Could be cool. Right?

No files, tests, or existing entry points are named. Start by reviewing how the project exposes commands and deciding whether seeding belongs in this library or an external library; done would mean supporting JSON or JS seed data through a working `pls seed ./seeds` command.

Written by the indexing model from the issue text.