Improvements to exposure extraction

[gouline]

Multiple requested improvements to exposure extraction:

• Optional grouping by type (e.g. question/123.yml) or collection (e.g. our_analytics.yml)
  • Resolves Sync metabase exposures as a list of files instead of a single file #102
• Sanitize exposure descriptions to replace {{foo}} with foo
  • Resolves Metabase exposures cause dbt to crash when referencing existing questions / variables because of jinja interpolation #148
• Sort exposures by name within each YAML file
  • Resolves Making exposures output idempotent #165
• Add inclusion collection name filtering (in addition to existing exclusion filtering)
  • Resolves Collection inclusion for exposures #201