publicPath is ignored when using with html-webpack-plugin

Thank you for this project!

# The problem
When using a `output.publicPath` in your webpack configurations, it is not used when inserting the primus generated file into the generated HTML file from `html-webpack-plugin`. [See demo here](https://github.com/newyork-anthonyng/primus-webpack-plugin-bug-demo).

```js
// webpack.config.js
module.exports = {
entry: "./app.js",

output: {
path: path.resolve(__dirname, "dist"),
filename: "bundle.js",
publicPath: "static/"
},

plugins: [
new PrimusWebpackPlugin(),
new HTMLPlugin()
]
};
```

And the HTML file looks like this:
```html

Webpack App

```

Notice how `static` is not prepended to the path for `primus-client.js`.

# The fix
A possible fix is to get the public path from `compilation.outputOptions.publicPath`, and prepend it to the filename that we are declaring [here](https://github.com/kkemple/primus-webpack-plugin/blob/master/index.js#L59-L61).

Let me know if this makes sense; I can create a Pull Request for this.

Start in index.js at lines 59-61, where the Primus client filename is declared, and inspect the webpack compilation output options used there. Use the provided webpack.config.js and demo to verify the generated HTML, and consider the issue's suggested publicPath behavior done when primus-client.js is emitted with the configured static/ prefix.

Written by the indexing model from the issue text.