Support comma separated strings for labels

[ketenshi]

Adds support for comma separated strings for the labels field so we can do this:

- id: label-maker
  run: |
     // generate labels list
     // labels="minor release,bug"
     echo "::set-output name=labels::$labels"

- uses: christianvuerings/add-labels@v1
   with:
   labels: ${{ steps.label-maker.outputs.labels }}
   env:
      GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}