Add support for complex config options

[mdreizin]

This PR adds ability to use use custom loader options:

{
    module: {
        loaders: [{
            test: /\.css$/,
            loaders: [
                'style',
                'css?config=customCssConfig'
            ]
        }]
    },
    customCssConfig: {}
}

Instead of #327, #315 I prefer to use recommended way to retrieve the loader config loader-utils#getloaderconfig.

@sokra Could you please review this PR?

[codecov-io]

Current coverage is 98.37% (diff: 100%)

Merging #348 into master will increase coverage by 0.02%

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[mdreizin]

cc @jhnns @bebraw

[bebraw]

What's the benefit over options?

[mdreizin]

@bebraw Here is some of benefits:

• it gives you ability to have shared config for group of xxx-loader
• it gives you ability override default options without modifying query/options of xxx-loader
• it gives you ability to have custom configs for some user cases
• it gives you ability to use recommended way to retrieve config of xxx-loader
• etc

Please see:

https://github.com/webpack/less-loader#less-plugins
https://github.com/jtangelder/sass-loader#sass-options
https://github.com/webpack/html-loader#advanced-options
webpack/loader-utils@7e86a18

[bebraw]

Gotcha. Looks like a good idea then so 👍 from me. I think @sokra can comment on the code once he has some time available.

[mdreizin]

@bebraw Thanks a lot.
It would be nice to have this feature as early as possible.
I'm looking forward to get feedback from @sokra.

[jhnns]

I thought we agreed that polluting the webpack config object is not a good idea? We also recently added config object validation to webpack@2, so this will not work with webpack 2, right?

[bebraw]

@jhnns Ok, I guess in that case we might to remove the feature from those other loaders to keep this consistent and document how to achieve the same effect without.

[sokra]

No longer allowed in webpack 2 as @jhnns said

it gives you ability to have shared config for group of xxx-loader

const sharedOptions = { ... }

it gives you ability override default options without modifying query/options of xxx-loader

options: {
  ...sharedOptions,
  key: value
}