Delay optimizer imports

[jednano]

Depending on configuration, this should get rid of some errors people are having. For example, #76 and #95. I was having the same problem, but I don't even have pngquant enabled!

These changes are backwards compatible, so no need for a major release.

{
    loader: 'image-webpack-loader',
    options: {
        pngquant: {
            enabled: false
        }
    }
}

[tcoopman]

Why does this fix the problem?

[jednano]

Because it now lazy loads the modules, so they only load if you use them. If you do enabled: false it never loads and therefore never throws the errors related to that module.