Allow deferring plugin execution so loaders have time to generate files

[Obi-Dann]

Hi,

We recently started using https://github.com/Jimdo/typings-for-css-modules-loader. It creates d.ts files for css-modules providing us with type checking and intellisense on css class names.
The problem is that the d.ts files are typically generated after fork-ts-checker-webpack-plugin is started. It means, the plugin either throws an error that a file is missing or checks an old version of d.ts file.

I suggest to add an async plugin hook fork-ts-checker-service-before-start so I can defer fork-ts-checker-webpack-plugin start up from another plugin.
If it sounds alright, I can create a PR implementing it.

I guess another option would be to add a flag startAfterFinishModules or startAfterCompile so the plugin will start type checking on compilation finish-modules or compiler after-compile

[piotr-oles]

Hi :)
The first option sounds more elastic for me, so please implement this one if it's not a problem.
PR's are always welcome :)

[Obi-Dann]

Awesome, thanks, I've created a PR #44

[piotr-oles]

Merged! I will release it as 0.2.8 :)