Make max releases configurable on command line

The max number of tries is [hardcoded to 10](https://github.com/99designs/cmdstalk/blob/fbd704da919d283643e673bd02efcf0221224b92/broker/broker.go#L30). It would be nice if that was configurable through a command line flag.

The hardcoded max tries is in broker/broker.go line 30. Look for where command-line flags are parsed, likely in main.go or a config package. Add a flag, pass it to the broker, and update the constant. Run the existing tests to ensure nothing breaks.

索引モデルが issue の本文から書いたものです。