Make each chain's `alerts` section optional

[bernatfp]

It shouldn't be required to specify the alerts section unless you want to override specific settings from the general level configuration. It will dramatically reduce the size of each config file and make it easier for new users to adopt and start getting value.

[gluckzhang]

Completed by #42, also updated the example-config.yml file. I will mention this breaking change again in the release note later.