use np to handle package releases

[chrisblossom]

This is an alternate to #144. Only merge one, close other.

This PR adds the package np to handle package releases. It will automate most everything you need to do when releasing an updated version. See the readme for a more detailed explanation on why.

Use the script npm run release instead of npm publish and follow the prompts.

Note: you might need to update your version of npm using npm install -g npm@latest

Changelog:

• Removed Node 6 support
• Removed webpack 2 support
• cleanOnceBeforeBuildPatterns use emit hook instead of compile
• Do not clean files if webpack errors are present during initial build
• Replaced default export with named export CleanWebpackPlugin

// es modules
import { CleanWebpackPlugin } from 'clean-webpack-plugin';

// common js
const { CleanWebpackPlugin } = require('clean-webpack-plugin');

[codecov-io]

Codecov Report

Merging #145 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #145   +/-   ##
=======================================
  Coverage   98.86%   98.86%           
=======================================
  Files           1        1           
  Lines          88       88           
  Branches       30       30           
=======================================
  Hits           87       87           
  Misses          1        1

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