Rake -> Thor

I prefer Thor (w/ or w/out a CLI) for a few reasons:
- You can use it like Rake, but it's also better as CLIs. That's better for Pacto to become a polyglot project (Ruby bindings available, but CLI + server can be used for non-Ruby projects)
- Much better handling of arguments and options so you can do things like "--strict", vs Rake's hacky way of passing parameters.
- Better documentation for commands w/ descriptions of available options/arguments.
- Easier to test - they're just Ruby classes.
- A lot of useful helpers (see Thor::Shell and Thor::Actions especially)