Update CHANGELOG.md for v0.12.0

[drewbanin]

That's a lot of commits!

TODO:

• Write docs for new features, linking to them in the changelog

Docs:

• Add support for tag configuration and selection (Add custom tags #1014)
• Add the set of schemas that dbt built models into in the on-run-end hook context (Provide a mechanism for granting usage to schemas in on-run-end #908)
• Add support for configuring the profiles directory with an env var (Allows the profile directory to be set with an environment var #1055)
• Add support for cli and env vars in most dbt_project.yml and profiles.yml fields (Render more config fields (#960) #1033)
• Fix for table clustering configuration with multiple columns on BigQuery (Fix clustering on multiple columns (BigQuery) #1013)