feat: optionally allow to push to other repositories

[fauno]

if you're pushing to your own repository, these gemspec options would prevent it, so i added the --unsafe flag

[luislavena]

Hello @fauno, thank you for your PR. Would you mind provide more context/background on this?

Thank you.

[fauno]

Sure! We're pre-compiling gems from rubygems.org and pushing them to our own private repository. Gems that have these options enabled on their gemspec would fail.

The docs recommend setting this to prevent accidentally pushing private gems to rubygems.org, but I've found some gems that set rubygems.org as the only allowed repository, so gem push after gem compile was being prevented.

https://guides.rubygems.org/publishing/#serving-your-own-gems