Make 'use_directory_urls' configuration option work from command line

**Common changes**

- There is a typo in the option name defined in config.py
- The value of 'use_directory_urls' is not passed through to SiteNavigation

**Notes**

I have not extracted the logic responsible for conversion from string to bool to a standalone function for the sake to preserve the current style. Most probably this code should be extracted to the independent function when other clients (I mean *'include_**' options which marked currently as 'TODO') are implemented.