Upgrade to latest Netlify Build

[ehmicky]

Thanks a lot for creating this Build plugin!

Netlify just released the latest version of Build plugins. Build plugins are currently enabled in the Netlify website for a small percentage of beta users.

This PR upgrades this plugin to the latest API. You can find the new documentation here.

Changes include:

• Add a bugs field in package.json
• Add a manifest.yml (including for default values)
• Remove name property from plugin
• Remove unused top-level function
• Rename constants.BUILD_DIR to constants.PUBLISH_DIR
• Thrown errors are reported as bugs by default and feature the name of the plugin, so errors can be directly propagated here (since they would indicate a bug in the plugin)
• Remove the if (inputs.outputFile) condition since this input has a default value and providing false would make the plugin do nothing

I have done some quick tests, but you might want to manually test this PR as well just to make sure :)

[ehmicky]

Is this ready to merge @DavidWells?

[DavidWells]

Sure. Please feel free to merge all updates to any plugins you see fit 😃