Replace adapter-transformers with Adapters

[calpt]

Closes #77.

This PR replaces the adapter-transformers code currently used in trankit by the newly released adapters package. Changes:

• Add adapters & transformers as dependency
• Small adaptations regarding adapter loading/ enabling logic
• Remove adapter-transformers code in adapter_transformers subpackage of this repo

Please note that some functionality used here is not yet released in the adapters PyPI package yet. For testing, please install Adapters like this:

pip install git+https://github.com/adapter-hub/adapters.git

Will release a new PyPI package before this PR is merged.