Modify loading of `appConfig` and `dataSourceConfig` to look for
the following files:
- app.json
- app.local.{js|json}
- app.{$env}.{js|json}
- datasources.json
- datasources.local.{js|json}
- datasources.{$env}.{js|json}
where $env is the value of `app.get('env')`, which usually defaults
to `process.env.NODE_ENV`.
The values in the additional files are applied to the config object,
overwritting any existing values. The new values must be value types
like String or Number; Object and Array are not supported.
Additional datasource config files cannot define new datasources,
only modify existing ones.
The commit includes refactoring of the config-loading code into
a standalone file.
Miroslav Bajtoš