"matrix" style monitoring

A `matrix` should allow you to configure a single file that can use multiple axes to simplify configuration.

For example:

```
matrix:
  - axis: [1,2,3]
  - variable: index
  - config:
      id: raspberry-pi-{{index}}
      test:
        timeout: 30s
        interval: 60s
        command: test.sh
```

It should be valid to nest `matrix` configurations. 