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
What if this path will be changed? I mean, what if tomorrow I'll decide to move example post in another folder? It's not cool that I should add a fix to init command too.