Collect all tools under one namespace in umd.

In webpack configs you not have standard naming convension for library name.
https://github.com/editor-js/image/blob/master/webpack.config.js#L45 here you set `ImageTool`,
https://github.com/editor-js/list/blob/master/webpack.config.js#L39 but here you set `Link`.
Webpack allow create [namespaces](https://github.com/webpack/webpack/tree/master/examples/multi-part-library#webpackconfigjs) for library. I think you must collect all tools under EditorJS namespace.

```js
// webpack.config.js
{
output: {
path: __dirname + '/dist',
publicPath: '/',
filename: 'bundle.js',
library: ['EditorJS', 'List'],
libraryTarget: 'umd'
}
}
```
Because EditorJS it's class you not get conflict of namespace, all tools will be collect as static methods.