Add support for multi compile mode

[PMCorbett]

Adds support for webpack multi compile mode by adding a cache object.

Fixes this issue: #13

Example Usage:

var StatsPlugin = require('stats-webpack-plugin');

var cache = {};

var config = [{
  ...
  plugins: [
    new StatsPlugin(
      'manifest.json', 
      {
        chunkModules: false,
        source: false,
        chunks: false,
        modules: false,
        assets: true
      }, 
      cache: cache
    )
  ]
  ...
}];

[PMCorbett]

This should also fix this issue mipearson/webpack-rails#19

[unindented]

Typo mulit?

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Comment about a minor typo, but looks good to me otherwise. If you fix it, I'll merge and publish a new version.

[PMCorbett]

Good catch, I've fixed that

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Pushed v0.5.0 with your changes. It'll be on npm once Travis does its thing.

[unindented]

Thank you for your help! 👏