Use webpack 4

[matrus2]

Rewrite tests and README.md to be compatible with webpack 4.

[infinateone]

Hi again Matrus!

I was having some trouble using contenthash with the plugin and saw you had this open, so before I do research you might have already done I wanted to see if you could confirm that building with Webpack 4 would resolve the issue.

Also, how is the update going?

[matrus2]

Hi @infinateone,

no significant work was done. Just try on your own :)