Add flags for git push command

[runfaj]

https://github.com/storybookjs/storybook-deployer/blob/6ee1d3e00596979257ef89bc05aede9ecf0f45aa/bin/storybook_to_ghpages#L70

Would be nice to add an option for adding additional git push flags. In our case, something like --no-verify would be crucial.