add `ddbt schema-gen` command to DDBT

The monzo dbt repo has a useful custom command dbt monzo schema-gen which automatically produces a yml schema file based on changes made to a model on someones dev branch, saving tedious yml text editing.

This version of ddbt will have similar functionality that runs in a fraction of the time. The command ddbt schema-gen model will produce a new yml file for new models, and an updated yml file for existing updated models.