add writeToDisk option to webpack plugin

[dlebedynskyi]

Summary

Adds option to emit file on build 158 by adding writeToDisk (default to false) to plugin options

Test plan

Test required app that runs 2 instances of webpack - client and server.
Client webpack instance need to generate file that is later used by server instance.
Razzle app maybe be a good candidate for that test

Example of usage

Default

// add plugin instance in webpack for client config
 plugins: [new LoadablePlugin()]

with emit file

// 👇 generate absolute path to target file 
const filename = path.resolve(root, 'dist/loadable-stats.json'); 

// add plugin instance in webpack for client config
 plugins: [new LoadablePlugin({filename:  writeToDisk: true})]

[gregberge]

OK for me, @marcneander can you confirm that it works for your razzle example?

[marcneander]

It works great 👍 I created my razzle example in his branch. Would have loved to have added it to this PR but I don't think it's possible so I created a new PR.

Let's get this merged and published. We're already running this code in our platform :)

[gregberge]

I can't merge it right now, documentation is not up to date. @dlebedynskyi can you update readme?

[willdurand]

// add plugin instance in webpack for client config
plugins: [new LoadablePlugin({filename: writeToDisk: true})]

I suppose, there is a comma after filename, no?

FWIW, I currently use this plugin to (I guess) do the same thing: https://github.com/gajus/write-file-webpack-plugin.

[dlebedynskyi]

I suppose, there is a comma after filename, no?
@willdurand my bad.

Thanks for merge and coma fix @neoziro.