CLI revamp

[srid]

• Advance to newer rib with revamped CLI
• Resolves Add a .neuron directory #78
• Resolves Feature Request: Add host option to rib serve #76
• Resolves Make rib commands fail when the zettelkasten doesn't exist #77
• Change CLI to be of the form: neuron -d ~/zettelkasten <cmd>
• Handle rib's cli args
  • neuron -d ~/zettelkasten rib -ws :8080 for serving (discard the s :8080 to watch and generate only).
  • neuron -d ... rib --output=/tmp/output to override output directory
• Refactor CLI code
• Add neuron open to open the locally generated output, using xdg-open on Linux and open on mac.
• Update docs & test

[srid]

Documentation changes: 9f40e33