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 do you think about rearranging commands to show user workflow? I mean to order them somehow like that:

             'init = holocron.ext.commands.init:Init',
             'build = holocron.ext.commands.build:Build',
             'serve = holocron.ext.commands.serve:Serve',