Add support for the --layouts flag

[shivrm]

As stated in the docs, Obraz does not support Jekyll's --layouts flag, or the layout_dir config option.
I would love to see this implemented in Obraz.

I've implemented something similar in my fork. Can I make a PR to merge this in? Feel free to suggest changes.🙂

Please note:

• Jekyll uses --layouts as the flag, and layout_dir as the config option name. I used layout_dir for both to avoid complexity.
• The default full_build_patterns config option seems to depend upon the layout directory. This might cause a bug in full_build_required. I'm working on fixing this.