Gadk

* `Artifact` now uses v2 of the download and upload actions. v2 includes a bug
fix for random upload failures.

* BUG FIX: `Step.env` was not rendered. It is now rendered after the `run` or `uses`
property.

* `gadk check` has been added to check if the generated workflow files match
what would be generated by running `gadk`. This is useful for CI jobs to ensure
that the `actions.py` file is modified, not the workflow files.
* `gadk --version` has been added to print the version of GADK.
* BUG FIX: `Workflow.on` does not error when specifying only `pull_request` or `push`.
* README has been expanded with instructions.

* Generated files now start with a comment to prevent manual modification.

* Abstractions of workflows are now supported:
python
class BaseService(Workflow, ABC):
abstractmethod
def concrete_data(self):
pass

class ServiceA(BaseService):
pass

In the example above, `ServiceA` will be detected as a workflow.
`BaseService` will not be rendered as expected; only concrete workflows
are rendered.
* The `workflow.on.branches` is now rendered before `workflow.on.paths`.

* Initial release.
* Basic features of workflows supported.
* `Artifact` abstraction included.