[CT-900] Complete CLI modeling (Top level API)

[iknox-fa]

User story:
As developer of dbt or a developer who would like to use dbt as a library, I would like to have an easily usable interface that models the user interactions at the CLI level (dbt compile, dbt run, etc).

Acceptance Criteria:
Extending the work done in the RAT, dbt-core has a new cli interface based on the click cli framework:

• It supports all regular user-facing use cases
• It fully and accurately describes each task and it's inputs (note, not the case today!)
  This work stops at the click.command that represents the task, or to be more explicit this work is only to model the existing cli not to fully implement successful task running.

Note: This should be on a feature branch for the time being.