Create new site with `lx init`

[chriskrycho]

Use a template to generate a new site with specified configuration.

Phase 0:

• [x] define basic structure of configuration (#2)
• [ ] create specific template(s) for site configuration

Phase 1:

• [ ] Load available templates (from someplace sane?) with the name lightning-<config>.yaml
• [ ] Create an instance it in (a) the current directory or (b) the directory specified by the user.

Phase 2:

• [ ] Simple readline-style UI which prompts the user for info about the site
• [ ] populate the template with the user-specified data

Phase 3:

• [ ] configuration validator (#12)