Automate publishing to npm

[rogermparent]

How does our npm package reflect changes to our repo?

Right now, it's manual publish- you have to cd into packages/gatsby-theme-iterative and run a yarn publish- ideally also changing the dependency version in the example sibling package but with patch upgrades semver will still use the latest version.

We definitely need to automate npm publishes, maybe not with every master merge but at least with a manual workflow run button that pushes the version bump commit.

Originally posted by @rogermparent in #22 (comment)