Create default directory structure with --init.

Addresses issues #451 and #194.

To be perfectly honest, I don't think this is _really_ ready to merge yet. I'd like to see all the directory / content creation moved to its own class. In addition, I'd like to be kicking out a sample post file complete with yaml front matter. However, I don't know when / if I'll ever get around to that, so I figure a pull request now is probably the best thing. Let me know what you think.