Holocron init command added

This commit adds init command to the holocron's cli. The init command
is meant to be the entry point of the user workflow. It creates an
example blogpost and a very basic _config.yml file in the current
directory.

Fixes #17
I know, you would say: "Why do we need to create another directory, if the example post already has a valid post path?". However, I would like to claim that creating example post in a path, which is represented by current data, looks cool to me and it gives you an obvious tip how to store the posts.