Keep push-manifest.json as single source of truth

[Prateek Bhatnagar]

Hi,
This code seems to be duplicated out of preact-cli: https://github.com/vinaypuppal/preact-cli-plugin-netlify/blob/master/index.js#L37-L42

Instead of this, you can just read push-manifest.json and convert it to netlify headers. This keeps you away from diverging from the preact-cli changes

[vínαч puppαl]

Implemented in #14