Feature request: Running Atlantis on a local directory

[syphernl]

We're using GitLab-CI for our Terraform projects, prior to implementing Terragrunt.
With Terragrunt a more elaborate setup is needed because of all the dependencies, which aren't easily handled within GitLab CI.

Right now this requires you to setup the Atlantis server somewhere and "hook it up" using a webhook in the project.
While this works, it requires a server that is accessible from internet and needs to be maintained.

Would it be possible to somehow introduce something like atlantis local which executes the workflows configured in atlantis.yaml on the local directory rather than it needing a full-blown server+webhook+git clone setup?
Something like this is easier to implement within the Gitlab-CI pipelines and gives you more control plus the ability to see the Atlantis output within GitLab.

[nitrocode]

Duplicate of #671