Unify flag/config story

Right now there are two different config entities:

0. [flags config](https://github.com/ContextLogic/eventmaster/blob/master/config.go#L4)
0. [config file](https://github.com/ContextLogic/eventmaster/blob/master/cmd/eventmaster/config.go#L12)

And some things can be set only by flag, and some only by config file. We should expose everything consistently.