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
1. My point is that you should prunt error message to `stderr`, not `stdout`. 

2. AFAIU, you have a problem that in case of `holocron init` we don't have `_config.yml` yet so we fail and exit, right? If so, we need to think how to handle this case. Generally, in my vision, it's ok if we exit in case something wrong with `_config.yml`.