Use npm/yarn scripts to facilitate versioning

[bcomnes]

Right now we use a bash script running npm scripts to generate the changelog:

https://github.com/netlify/build-image/blob/12783d4d177a66bec5a23ad1097ea7bd02931128/gen-changelog.sh

There are a couple of problems with this approach:

• Global dependencies via npm. Requires anyone performing the release process to install a global tool.
• Requires user input: You need to run ./gen-changelog.sh -v vX.X.X. This requires remembering which flags and manual version formatting which is troublesome to remember.

There are a few things we could improve on that flow:

• Add a package.json to the project and add a couple of release scripts.
• Eliminates global peer dependencies (project local)
• Auto-version bumping. All you have to do is npm version [major, minor, patch] && npm run release and it creates the changelog, creates the version commit and tag, pushes it to github, and updates gh-release with the new section.
• Ensures lots of release steps are automated and done consistently.
• Works with npm or yarn.
• We can wrap in a Make file if we want.

We did something similar for the cli and I really enjoy the workflow: https://github.com/netlify/cli/blob/master/package.json#L26-L27