Add transform function

[jahredhope]

This change is an attempt to address the below issue. Depending on any feedback on this issue this change may not be needed.

closes: #121

Transform function

Adds a transform function that allows locals to be transformed into the variables expected by render function.
Transform function runs per render path.

Example usage:

const entries
new StaticSiteGeneratorPlugin({
  transformLocals: (locals) => {
  const mainChunkAssets = locals.webpackStats.assetsByChunkName.main
    .map(value => `${publicPath}${value}`);

  const clientJsEntries = mainChunkAssets
    .filter(value => value.match(/\.js/))

  const clientCssEntries = mainChunkAssets
    .filter(value => value.match(/\.css$/))

    return { ...locals, clientJsEntries, clientCssEntries}
  }
})

[coveralls]

Coverage remained the same at 100.0% when pulling 79eb95c on jahredhope:add-mapping-function into ee7b961 on markdalgleish:master.

[jahredhope]

Unable to get tests to pass.
Attempted completely fresh install, passed locally but CI fails.

This change now limits tests to comparing HTML files. But would appreciate advice how to fix differences between CI and local.

[jahredhope]

After further discussion it seems it may be cleaner to deprecate locals as a object and for the user to use the previously described transformLocals behaviour for any use of custom values.
This change now represents this behaviour.

[markdalgleish]

Probably worth adding webpackStats to this.