Add custom tags

[drewbanin]

Fixes #311

Specify tags for a model:

{{
  config(tags=['nightly'])
}}

-- OR:

{{
  config(tags='nightly')
}}

select ..

or:

models:
  snowplow:
    # Set tags as a string
    tags: bi

  my_project:
    clickstream:
      # Set tags as a list
      tags: ['nightly', 'crm']

    advertising:
      tags: ['nightly']

Select models with a tag:

$ dbt run --model tag:nightly

Select using a mix of tags, fqns, and parent/child selectors:

$ dbt run --model tag:nightly+ salesforce.*+

As a result of this PR, selectors can also explicitly use the fqn: selector type to select models by their full qualified names:

$ dbt run --model fqn:snowplow_sessions
# equivalent to:
$ dbt run --model snowplow_sessions

TODO:

• add test for setting tags in dbt_project.yml
• good error message when a bad selector is provided
• support setting tags as a string

Error message when a bad selector is provided

$ dbt run --model tags:10
Found 1 operations, 179 macros, 1 analyses, 1 seed files, 3 tests, 3 models, 1 archives
The 'tags' selector specified in tags:10 is invalid. Must be one of [tag, fqn]
* Spec='tags:10' does not identify any models and was ignored

WARNING: Nothing to do. Try checking your model configs and model specification args

[abelsonlive]

nice! very excited about this :)