Add draft for stringVariable #533

[TobiasNx]

@dr0i could you help me with this so I could move on?

[dr0i]

Please stick to our contributing conventions:

When submitting a pull request use a meaningful title and use closing keywords in the pull request description to reference the issue it is meant to resolve.

(see https://github.com/metafacture/metafacture-core/blob/master/CONTRIBUTING.md#pull-requests)
At least the proper referencing to the parent issue you can do that.