Add DbtSnapshotOperator, test and update Readme

[faouzelfassi]

Add DbtSnapshotOperator: that calls dbt snapshot #12

[rliddler]

Thanks for the PR @faouzelfassi !

I've noticed there seem to be a lot of formatting changes (I guess automatically added by your linter?) if you could remove those so it's a PR of just the required changes that would be great!

I believe we have a line limit of 110 specified in our flake8 for this project.

[prratek]

The dbt snapshot CLI command supports a selector that's used like dbt snapshot --select some_model. To support passing in a selector, would we need an additional parameter for the DbtCliHook?

[andrewrjones]

I wasn't able to fix the formatting on this PR so have created a new PR from @faouzelfassi's commits that is ready for review: #22. Closing this one.

@prratek Not sure how best to support the additional options, but would probably suggest trying to change the signature of run_cli to take optional arguments. Created an issue for that here: #23