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
If i don't remove it, using of the `holocron init` is impossible. When we run the command it first checks for the config file, and if it fails to find it, the execution is terminated.  

I can leave the `show_error_and_exit` method, but we still need to remove or replace the `sys.exit(exit_code)` with something else.