Pass custom settings to the DB's SESSION (#1063)

- allows queries to refer to current_setting('app.settings.foo') to retrieve variables
- useful for 12-factor apps (app data can be in environment)
- provides workaround for AWS Relational Database Service (RDS) not
  allowing `ALTER DATABASE SET 'app.[KEY]' TO '[VALUE]'` on database.

CHANGELOG.md

@@ -17,6 +17,7 @@ This project adheres to [Semantic Versioning](http://semver.org/).
 - Computed columns now only work if they belong to the db-schema - @steve-chavez
 - To use RPC now the `json_to_record/json_to_recordset` functions are needed, these are available starting from PostgreSQL 9.4 - @steve-chavez
 - Overloaded functions now depend on the `dbStructure`, restart/sighup may be needed for their correct functioning - @steve-chavez
+- The configuration (e.g. `postgrest.conf`) now accepts arbitrary settings that will be passed through as session-local database settings. This can be used to pass in secret keys directly as strings, or via OS environment variables. For instance: `app.settings.jwt_secret = "$(MYAPP_JWT_SECRET)"` will take `MYAPP_JWT_SECRET` from the environment and make it available to postgresql functions as `current_setting('app.settings.jwt_secret')`. Only `app.settings.*` values in the configuration file are treated in this way. - @canadaduane
 
 ## [0.4.4.0] - 2018-01-08