Output schema

from @ConnorGraham
---
BitOps does a poor job passing properties from one executable to the next. This is currently possible by hardcoding the property as an `option` in `bitops.schema.yaml`:
```
type: object
properties:
cli:
...
options:
type: object
properties:
fetch-kubeconfig:
type: boolean
export_env: FETCH_KUBECONFIG
default: true
```
This is not scalable as it would require modifying `bitops.schema.yaml` for every new output value. The responsibility of defining outputs should be pushed to an ops repo in `bitops.config.yml`

**Sample `bitops.config.yaml`**
```
terraform:
cli: {}
options: {}
outputs:
- name: kubeconfig
export_env: KUBECONFIG
- name: my-favourite-color
export_env: FAV_COLOR
```
The above config.yml would instruct bitops to look for `kubeconfig` and `my-favourite-color` in a `terraform output` command (e.g. `terraform output kubeconfig`) and set it to the requested ENV var.