Allow no specification for both ´nav´ and ´docs_dir´

[dariocurr]

In order to solve #81, I edited the code to support the missing key docs_dir in the included mkdocs.yml file.

More in detail, if neither nav nor docs_dir keys are found on the included mkdocs.yml, docs_dir is set at its default, i.e. docs

There is no need to add another log message in parser.py, since if the folder is not found, mkdocs itself would raise the following error: .../docs isn't an existing directory

[dariocurr]

Thank you for your useful plugin! :nerd_face:

Happy and glad to contribute!