Adding New Configuration Parameters

With new setting manager, you can define new configurations and modify them in src/include/settings/settings.h, using macros SETTINGS_* which is similar to those in gflags.

Define a new configuration

`SETTINGS_int(name, description, default_value, is_mutable, is_persistent)`

`SETTINGS_int(port,
              "Peloton port (default: 15721)",
              15721,
              false, false);`

More details can be found in src/settings/README.md