Add the ability to specify an editor in _config.yml

This PR adds an 'editor' subkey to the configuration options for poole.
Using this subkey to specify an editor will cause that editor to open when
'poole post' or 'poole draft' is invoked.

For example, my config.yml looks like
```yaml
poole:
    word_separator: "-"
    editor: "subl"
```

Which will cause any posts or drafts I create to immediately open in Sublime Text upon creation.