Load datasources and app cfg from multiple files

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.