Introducing test utils

Not a problem this time, but I would like to share an idea. It's something I was working on during last weeks.

I've seen and use a lot of nice plugins to Hexo, but only few are automatically tested. With Hexo is very easy to write test for plugin, but the boilerplate code needed for it may discourage to try.

To improve the situation and ease adding specs for plugins I came up with [hexo-test-utils](https://github.com/ertrzyiks/hexo-test-utils). It's a Sandbox and set of utilities. Currently, it has only those I needed for https://github.com/hexojs/hexo-filter-responsive-images . It could and should have way more.

The documenation for the current API is [here](https://ertrzyiks.github.io/hexo-test-utils/)

@NoahDragon if you like the idea, I will move the repo to the organization