Support using plugin instance in multiple webpack configs

[th0r]

Fixes #10

Currently if you use this plugin in multi-config webpack configuration it will generate/validate dll bundles N times in parallel where N is the number of configs.

This PR solves it: just create one instance of DllBundlesPlugin and use it in multiple configs.
It won't generate/validate bundles if this process has already been started.
For example, can be helpful during migration from Angular 1 to Angular 2 with something like that:

webpack.config.js:

var vendorBundlePlugin = new DllBundlesPlugin({
  dllDir: ...,
  // Libraries that can be used in both ng1 and ng2 apps
  bundles: {
    vendor: ['angular', 'lodash', 'moment']
  },
  webpackConfig: ...
});

var ng1Config = {
  ...
  plugins: [vendorBundlePlugin],
  ...
};

var ng2Config = {
  ...
  plugins: [vendorBundlePlugin],
  ...
};

return [ng1Config, ng2Config];