Add `webpack` to `devDependencies`
Add webpack config and `main.js`

Running `node_modules/.bin/webpack` will compile `main.js` to
the `dist` directory, including a hash in the filename for caching.
Working build script

Running `bin/build.js` will:

1. Delete the build directory `dist` if it exists
2. Run the webpack build
3. Copy `index.html` to the build dir
4. In `index.html`, replace "main.js" with the correct hashed filename
Move main JS and HTML files to `src` dir
Merge branch 'build'