Allow initializing enummer with hash

[GabrielNagy]

Similar to the Rails enum behavior, allow initializing an enummer with a hash where the values correspond to the indices of the bits that map to the flag.

This is a more robust and backwards-compatible approach to enums, as the developer doesn't have to worry about migrating historical data if an enum value is removed.

I also took the liberty of running standardrb on the entire project, seeing as the gem is part of the development dependencies.

Thanks again for this useful gem!