support custom version format

[kerryChen95]

new AppCacheWebpackPlugin({
  comments: [
    '[hash][YYYY-MM-DD HH:mm:ss]', // compilation hash, compilation time in moment.js format
    'some other comments'
  ]
})

Compilation hash info. is easy to lose, we have to record each release's compilation somewhere, so we need compilation time to known it's which version.
And when I use html-webpack-plugin to generate html file, I just modify the html file template, the compilation hash does not change, maybe it's html-webpack-plugin's duty to update compilation hash, but there are still some edge cases to need additional comments in appcache manifest file.