npm version support, precheck state, export environment

- custom npm version bootstrap
- skip semver for specific versions
- quiet logging unless there are errors
- `npm install` `npm update` and `npm dedupe` for deterministic builds
- devDependencies controlled via `npm_config_production` env var
- heroku env vars available throughout build process
- exports `NODE_HOME` and extends `PATH` for node bins (multi-buildpack)
- determines state upfront and offers suggestions
- protips link to specific articles