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'm not sure that it works. Try to install holocron in separate virtualenv and run `holocron init`. Please note, you need to run `python setup.py install`, not `python setup.py develop`.