Add os for tests

Add a field which defines on which `os` a test can be executed, i.e.:

```
tests:
    echo hello:
        os:
        - darwin
        - linux
```

or maybe this is even better:

```
config:
    os: linux # Define default os
tests:
     print hello:
         cmd:
             osx: echo hello osx
             windows: echo hello windows
             linux: echo hello linux
```