Have a link checker in the CI

[budziq]

We seriously need a link checker with following properties:

• check if internal links are rendered correctly (it has to understand markdown )
• understand the mdbook's {{include links.md}} syntax
• optionally makes requests to the actual links testing for errors (this would not be used in CI as all the environments of that type forbid networked requests)
• standalone cli tool receiving list of files or directory as an input or be a part of the mdbook itself https://github.com/azerupi/mdBook/issues/356
• ideally it would be incorporated to our CI (this is not a must for the first prototype)