Accept a YAML array for labels

Hey there! :wave:

This is my first time using slurper and I must admit I didn't read the documentation very well, so I wrote a bunch of stories with labels like this:

```yaml
labels:
  - faith
  - hope
  - love
```

Instead of like this:

```yaml
labels: faith,hope,love
```

Anywho I think supporting both makes for a really nice & predictable interface especially for those irritating guys who don't RTFM like me :wink:

Have at it! :heart: