add ability to specify a migration environment key

This PR allows you to do this
```
./node_modules/knex/bin/knex migrate:latest --environment=development
```
or
```
./node_modules/knex/bin/knex migrate:latest -e=development
```

Thus allowing for this syntax in your config file

```js
module.exports = {
  "test": {
    ...
  }
  "dev": {
    ...
  },
  "production": {
    ...
  }
}
```

this allows you to keep your config centralized
and dynamic based on your environment.