Should not throw on missing modules

[mastilver]

It should just log a warning instead

[shirotech]

Just the plugin I was looking for, I'm surprised it does not exist anywhere else yet. Nice work so far 👍

When a module is not found I'm getting this:

throw new Error(`'${moduleName}' is not available through cdn, add it to https://github.com/mastilver/module-to-cdn/blob/master/modules.json if you think it should`);

Would it be possible to add an option which can be additional to the modules.json file, rather than needing to do a pull request every time it doesn't find it.

i.e.

new ModulesCdnWebpackPlugin({
  additional: {
    "vue": {
      "var": "Vue",
      "url": "https://unpkg.com/vue@[version]/dist/vue.runtime.min.js"
    }
  },
  modules: ['vue']
})

This would make it much more flexible and less opinionated, what are your thoughts?

Cheers.