feat: allow disabling push to remote

[drlkf]

This pull request allows configuring a new boolean option called push_remote which defaults to true, which determines whether the changes are pushed to the remote or not. This is a feature that what requested in #532 that I would like to use in my workflow. I know it's not exactly in line with the philosophy of semantic-release, but just like this plugin, sometimes this is what you need to do.

I'm not at all proficient in the Javascript ecosystem, I went for the obvious test suite, I might have not seen other places where I should test stuff. Let me know if I've done something wrong or forgotten something.

cheers!