Feat/new resolver

[jhnns]

This PR introduces two important changes:

• It lets you decide between the basic Less resolver and the webpack resolver. If you pass a paths option, it will disable webpack's resolver and use the given paths as include paths. You can also set an empty paths array to disable webpack's resolver without specifying any include paths.
• If the webpack resolver is used, you are allowed to import non-less files given you have configured the loader pipeline correctly such as

// webpack.config.js
module.exports = {
    ...
    module: {
        rules: [
            {
                test: /\.js$/,
                // if the requesting module is a less file, than use this loader pipeline
                issuer: /\.less$/,
                use: [{
                    loader: require.resolve("./jsToLessLoader.js")
                }]
            },
            {
                test: /\.less$/,
                use: [{
                    loader: "css-loader"
                }, {
                    loader: "less-loader"
                }]
            }
        ]
    }
};

As a breaking change, an undocumented root option was removed. This is unlikely to break anything since it was undocumented, but it's still a breaking change.

[codecov]

Codecov Report

Merging #169 into master will increase coverage by 0.12%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #169      +/-   ##
==========================================
+ Coverage   98.61%   98.73%   +0.12%     
==========================================
  Files           7        7              
  Lines          72       79       +7     
  Branches        4        5       +1     
==========================================
+ Hits           71       78       +7     
  Misses          1        1

Impacted Files	Coverage Δ
src/getOptions.js	100% <100%> (ø)	✅
src/stringifyLoader.js	100% <100%> (ø)
src/createWebpackLessPlugin.js	100% <100%> (ø)	✅

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