Add transactional json logging for dbt invocations

[drewbanin]

Describe the feature

This PR adds a json logger to the dbt project. When json logging is used, dbt should emit "transactional" log lines that report on the state of the run. From these logs alone, external processes should be able to paint a UI with a superset of the stdout logging that dbt presents on the CLI. This includes:

• The current state of the dbt project (internal, parsing, running)
• The current state of the resources that have run, or are running:
• state: running, complete
• status: running, cancelled, success, fail, error, etc.
• status_message: (the thing we print on stdout at resource completion)
• timing: (started_at, finished_at)
• error: if an error occurred in the scope of a resource, the error (as a string)

Implementation notes:

• log lines that occur during each of internal, parsing, and running should be tagged with an attribute, run_state (or similar), so they can be collected and rendered together
• internal is for log lines that occur outside the scope of parsing and running
• these log lines should be easily identifiable from the log stream, eg with the channel attribute, or similar
• we can either cram this information into the extra dict, or we can represent this data some other way if that makes more sense.