[REQUEST] Support jekyll blog nested

We need to support the situation when a blog is not at the root of the project. Maybe we can enforce to possibilities: the root or a 'blog' folder, or maybe we can implement some parser to know where a blog is inside a repo (maybe we can support a configuration file inside the repo like `.blogcop.yml` with the path of the blog which is a really common pattern and fallback to `/` if not present).

That way we can install blogcop on projects that has a jekyll blog inside but are not just a jekyll blog.